If the string hits your arm your draw length could possibly be too long for ya .Try shortning up the draw length.It could also be the grip U have on the bow .After a few strikes with the string on your arm you'll soon figure out what's the problem thenail
String slap is a matter of being over drawn or a matter of improper elbow position. You have to turn that elbow to the ground or "raise the elbow" as the saying goes. Makes for proper form and better shooting anyhow so if your feeling the sting its time to address a form change or drawlength change quick like.
Observation.... You see alot of guys wearing arm guards anyhow so what gives are they all shooting equipment that doesnt fit them or in bad form. Answer No not really. Alot of guys are using that arm gaurd as a clothing gaurd so as to keep the string from hitting bulky clothing. Also it isnt uncommon to find your self in an odd position in the field and wel maybe that form isnt quite right and that elbow dips. Some people never get over string slap but I think many of the folks that do have a problem with it could solve it with some form coaching and proper equipment sizing.
As most everyone else has stated.
Draw length is long and elbow in
Keep a slight bend in the bow arm with the elbow turned out.
I don't have a problem with the compounds because there is a set draw length that won't allow me to over straighten the bow arm.
With the traditionals I wear an arm guard.
When I'm hunting. I wear a guard. Just because.
If you're lucky enough to be on private land and can leave your stand up during the off season you can tie an old piece of cloth or something of the sort to your stand. I like to use an old camo bandana that I have. That way the deer get used to the movement and eventually don't bother even looking up when there is slight movement. This may just give you the time to draw your bow or raise your gun without being detected.