http://www.jcs-group.com/military/rank/salute.html
Found this earlier today about saluting protocol. Reagan started this, and had asked one of his top military guys if it was okay as a civilian president to salute those in military. The response was something like “Heck, your Commander in Chief - you can salute whoever you want to!”
The link covers some of the possible problems. Such as carrying something. If possible, you are to hold the object in your left hand and salute with your right. If your right arm can’t salute (injury), a left-handed salute is okay.
If both arms are full, a nod and a “hello” is okay. Rules vary between the military branches as well.
Interesting read at the link. Talks a lot about the custom of giving a silver dollar to the first person to salute you after becoming an officer!