What are the causes of string slap and what are the cures?
I get string slap a lot- most of the time.  I wear a arm guard, but sometimes it even hits my cheek.  
This is the case whether I'm shooting a board bow with a self made B50 endless string, or my Cari-bow Slynx with a flemish from the maker (I don't what kind of material, but it shoots eally nice).  Some strings have yarn silencers; the Cari-bow came with rubber whiskers.  Some have almost no twists, some are really twisted. It also doesn't seem to matter how good of a shot I make- hit the mark or fly off into the sunset, I get bit.
Is this form/ release or a equipment issue?

Any time I'm shooting a low-brace-height bow I make sure I have one on. I used to have a D-type longbow that raised an ostrich egg in between my wrist and forearm before I finally gave up and decided I just couldn't shoot it without a guard. Didn't make any difference how far out I twisted my bow elbow, it still got me. I can normally shoot my recurves without an arm guard, but just wear one for that one time in a hundred. Man that smarts!

Since it's happening with all of the bows you shoot, I'd say it's form related. If you do a search in the "shooter's form" forum, I posted the same question quite a while back and Moebow replied with all the answers I needed. I can't explain it anywhere near as well as he did, so I won't try. But I will say that for me, my bow hand was the culprit and after doing what Arne said to do, it stopped happening to me. I'm posting this from my phone, otherwise I'd try to elaborate.

Hope this helps some.

Hits your cheek ?
Do you draw clear back to your ear ?

For me personaly its usually brace height. when I get the brace up to 7" I dont have a problem any more.  Also try opening up your stance a little.  For your cheek, if you're using a tab make sure it is trimed down. Extra tab flap hits my cheek.  Good luck

I hit my cheek with my fingers on release sometimes, but I've never hit it with the string.  I was having the same issue with wrist slap though and I posted a question about bow hand grip.  I got a good answer, changed by grip on the bow and now I don't even wear an arm guard.  I'll see if I can find the post

edit:
I found it.  This is a link to a thread I was directed to.  Moebow's advice cured my string slap issues

http://tradgang.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=6;t=005763#000000

Don't view string slap as a bad thing.  If you are hitting your arm...you are doing something wrong form wise.  When you quit hitting your arm..you are doing something right. Adjust the way you grip the bow until you are no longer hitting your arm and not only will your arm feel better but your shooting will improve! This is actually one reason why I greatly prefer bows with the limbs mounted on the belly like Black Widow, Habu, etc.  Because of the high brace height, I never hit my arm unless I really do something wrong.  In a hunting situation this is one less thing I have to worry about.  Especially when I have on a bunch of heavy clothing while sitting in a cold November treestand.

can depend on torsional ridgidity of the limbs too. i shoot my border at 6 3/4" brace and no problems.

I get it always because I have deformed elbows.  I ordered a bracer from 3Rivers because it's so bad.  I can't change form to correct it because my elbows are stuck the way they are.

I don't draw to my ear, the string hits my cheek near my mouth.
I use a glove, and it is definetely the string.
My brace hieghts are between 6 3/4 and almost 8 on one long bow.
Sounds like I need to spend some time on MoeBow's advice and see what I can do.
What is most frustrating is that I don't want to change my form since I'm hitting more where I want than a while back.  I'll take a string bite, even in the face, if it means I hit a double lung shot!

Too much weight ( heavy string silencer usualy) on the string can be one case of string slap too.

how about "too many strands" of strings?  It wouldn't explain the flemish on the Caribow, but I'm shooting about 50-56# bows with 16 strands of b50.

16 stands of dacron is good but if you have to much twist on it,it's not good because this is add elasticity,a string with too much elasticity can be a cause of string slap, it's same for the modern fast flight material,usualy the correct amount of twist on string is 1 or 2 twist max per inch.
Nock to tight can be also a another cause to.

UPDATE:
Thanks for the responses- it is my grip!  It is a form issue, mostly.  The bow grip link for Moe Bow was the ticket and no more whacks to the face (occasional collapse gives a arm bite)!
Definitely too tight of nocks too, but not the problem.
Happy Shooting!!

overstretching your bow-arm can also cause it