this is mostly for longbow shooters, but is it common for the string to hit your arm. i shoot a Hill bow with a 61/4 inch brace height.

No.  You just have to make some adjustments in your syle until it doesn't.

i had this same problem about a month ago. i just started with the longbow and it was hitting my arm almost every shot. i fixed my problem by gripping my bow more to the side. it feels awkward at first, but it fixed the problem.

A couple of things.  

Check your stance.  If you shoot right-handed, your right foot should be pretty close to parallel to the target.  Stand with an open stance, your body is between 30 and 45 degrees of parallel.  I know this sounds dumb, but I don't know how else to write this.

I think as VinnieB was saying, your bow is in that 'V' section between your thumb and hand.  Maybe even more thumb than hand.

Make sure your elbow is rotated out.  Grip anything that's virtical and solid and practice.(The virtical edge of a door works.)

Cant your bow to about 1:00 - 1:30.  AM or PM, doesn't matter.    

Check your brace height.  If your bow is suppose to be from, say, 6" to 7", try closer to 7".

I really, really, really dislike wearing an armguard.  Hot and uncomfortable are two that come to mind.  So I will do anything I can to NOT wear one.  When I get kissed, I know I've done something wrong.

Thanks all for the tips.  I don't mind wearing an arm guard personally, however I find that the string is constantly slapping me just above the guard on the inside of my elbow.
I've looked all over for more full coverage guards but can't find a thing. So, I guess I need to strap on an ace bandage and work on my form.

My longbow gets my attention when I mess up.  Here are a few things to try.  (1) Be sure you aren't locking your elbow.  Your bow arm should be very slightly bent.  (2) Try a slightly looser grip on the bow.  (3) Try facing the target just a tad more.  (4) Be sure you aren't plucking the string.  Any or all of these things might help.  It's hard to shoot well when you are anticipating getting slapped by the string!

Good Luck!
John