United States of America
Practice Relating to Rule 62. Improper Use of Flags or Military Emblems, Insignia or Uniforms of the Adversary
The US Field Manual (1976) states: “It is especially forbidden to make improper use … of the national flag, or military insignia and uniform of the enemy.” 
United States, Field Manual 27-10, The Law of Land Warfare, US Department of the Army, 18 July 1956, as modified by Change No. 1, 15 July 1976, § 52.

The manual adds:
In practice, it has been authorized to make use of national flags, insignia, and uniforms as a ruse. The foregoing rule (HR, art. 23, par. (f)) does not prohibit such employment, but does prohibit their improper use. It is certainly forbidden to employ them during combat, but their use at other times is not forbidden. 
United States, Field Manual 27-10, The Law of Land Warfare, US Department of the Army, 18 July 1956, as modified by Change No. 1, 15 July 1976, § 54.
[emphasis in original]
The manual also states:
Members of the armed forces of a party to the conflict and members of militias or volunteer corps forming part of such armed forces lose their right to be treated as prisoners of war whenever they deliberately conceal their status in order to pass behind the military lines of the enemy for the purpose of gathering military information or for the purpose of waging war by destruction of life or property. Putting on civilian clothes or the uniform of the enemy are examples of concealment of the status of a member of the armed forces. 
United States, Field Manual 27-10, The Law of Land Warfare, US Department of the Army, 18 July 1956, as modified by Change No. 1, 15 July 1976, § 74.

The US Air Force Pamphlet (1976) incorporates the content of Article 23(f) of the 1907 Hague Regulations and adds that the prohibited improper use of the enemy’s flags, military insignia, national markings and uniforms “involves use in actual attacks”. 
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, §§ 8–2 and 8-6(c); see also § 8-3(d).

The manual further specifies:
Members of the armed forces of a party to the conflict and members of militias or volunteer corps forming part of such armed forces lose their right to be treated as prisoners of war whenever they deliberately conceal their status in order to pass behind the military lines of the enemy for the purpose of gathering military information or for the purpose of waging war by destruction of life or property. Putting on civilian clothes or the uniform of the enemy are examples of concealment of the status of a member of the armed forces. Ground forces engaged in actual combat, in contrast to ground forces preparing for combat, are required to wear their own uniform or distinctive national insignia.

While combatant airmen are not absolutely required to wear a uniform or distinctive national insignia while flying in combat, improper use of the military insignia or uniform of the enemy is forbidden. Consequently, airmen should not wear the uniform or national insignia of the enemy while engaging in combat operations. Military aircraft, as entities of combat in aerial warfare, are also required to be marked with appropriate signs of their nationality and military character. 
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, §§ 7–2 and 7-4.

The US Naval Handbook (1995) states:
At Sea. Naval surface and subsurface forces may fly enemy colors and display enemy markings to deceive the enemy. Warships must, however, display their true colors prior to an actual armed engagement.
In the Air. The use in combat of enemy markings by belligerent military aircraft is forbidden.
On Land. The law of land warfare does not prohibit the use by belligerent land forces of enemy flags, insignia, or uniforms to deceive the enemy either before or following an armed engagement. Combatants risk severe punishment, however, if they are captured while displaying enemy colors or insignia or wearing enemy uniforms in combat.
Similarly, combatants caught behind enemy lines wearing the uniform of their adversaries are not entitled to prisoner-of-war status or protection and, historically, have been subjected to severe punishment. It is permissible, however, for downed aircrews and escaping prisoners of war to use enemy uniforms to evade capture, so long as they do not attack enemy forces, collect military intelligence, or engage in similar military operations while so attired. As a general rule, enemy markings should be removed from captured enemy equipment before it is used in combat. 
United States, The Commander’s Handbook on the Law of Naval Operations, NWP 1-14M/MCWP 5-2.1/COMDTPUB P5800.7, issued by the Department of the Navy, Office of the Chief of Naval Operations and Headquarters, US Marine Corps, and Department of Transportation, US Coast Guard, October 1995 (formerly NWP 9 (Rev. A)/FMFM 1-10, October 1989), § 12.5.

The US Naval Handbook (2007) states:
12.5 Enemy Flags, Insignia and Uniforms
12.5.1 At Sea
Naval surface and subsurface forces may fly enemy colors and display enemy markings to deceive the enemy. Warships must, however, display their true colors prior to an actual armed engagement.
12.5.2 In the Air
The use in combat of enemy markings by belligerent military aircraft is forbidden.
12.5.3 On Land
The law of land warfare does not prohibit the use by belligerent land forces of enemy flags, insignia, or uniforms to deceive the enemy either before or following an armed engagement. Once an armed engagement begins, a belligerent is prohibited from deceiving an enemy by wearing an enemy uniform, or using enemy flags and insignia; combatants risk severe punishment if they are captured while displaying enemy colors or insignia or wearing enemy uniforms in combat.
Similarly, combatants caught behind enemy lines wearing the uniform of their adversaries run the risk of being denied prisoner-of-war status or protection and, historically, have been subjected to severe punishment. It is permissible, however, for downed aircrews and escaping prisoners of war to use enemy uniforms to evade capture, so long as they do not attack enemy forces, collect military intelligence, or engage in similar military operations while so attired.
Captured enemy equipment and supplies may be seized and used. Enemy markings, however, should be removed from captured enemy equipment before it is used in combat. 
United States, The Commander’s Handbook on the Law of Naval Operations, NWP 1-14M/MCWP 5-12.1/COMDTPUB P5800.7, issued by the Department of the Navy, Office of the Chief of Naval Operations and Headquarters, US Marine Corps, and Department of Homeland Security, US Coast Guard, July 2007, §§ 12.5, 12.5.1, 12.5.2 and 12.5.3.

Under the US War Crimes Act (1996), violations of Article 23(f) of the 1907 Hague Regulations are war crimes. 
United States, War Crimes Act, 1996, Section 2441(c)(2).

In the Skorzeny case before the US General Military Court of the US Zone of Germany in 1947, the accused, German officers, were charged with participating in the improper use of American uniforms by entering into combat disguised therewith and treacherously firing upon and killing members of the US armed forces. The Court did not consider it improper for German officers to wear enemy uniforms while trying to occupy enemy military objectives. There was no evidence that they had used their weapons while so disguised, so the accusation of war crime was rejected. All the accused were acquitted and the Court did not give reasons for its decision. 
United States, General Military Court of the US Zone of Germany, Skorzeny case, Judgment, 9 September 1947.

It was reported that, during the December 1944 Battle of the Bulge, the US army executed 18 German soldiers apprehended in US uniforms on charges of spying. 
W. Hays Parks, “Air War and the Law of War”, The Air Force Law Review, Vol. 32, 1990, p. 77, footnote 259; Peter Rowe, “The Use of Special Forces and the Laws of War. Wearing the Uniform of the Enemy or Civilian Clothes and of Spying and Assassination”, Revue de Droit Militaire et de Droit de la Guerre, Vol. 33, 1994, p. 217.

In 1987, the Deputy Legal Adviser of the US Department of State affirmed: “We do not support the prohibition in article 39 [of the 1977 Additional Protocol I] of the use of enemy emblems and uniforms during military operations.” 
United States, Remarks of Michael J. Matheson, Deputy Legal Adviser, US Department of State, The Sixth Annual American Red Cross-Washington College of Law Conference on International Humanitarian Law: A Workshop on Customary International Law and the 1977 Protocols Additional to the 1949 Geneva Conventions, American University Journal of International Law and Policy, Vol. 2, 1987, p. 425.

According to the Report on US Practice, the Skorzeny case is the leading authority for the US armed forces. It adds that it is the opinio juris of the United States that the use of enemy uniforms is a lawful ruse of war as long as they are not used in actual combat. 
Report on US Practice, 1997, Chapter 2.6.