Reinforcers (rewards):  I like to use as many natural reinforces as possible, and find reinforcers that are reinforcing.  The main things that I find myself using (and the students asking for) are:

a) Detention passes.  Four equal a Saturday school pass.  They can use them for any 
teacher's detention. 

b) Late passes.  This is something they can use to not get a tardy from me, or have me write  a note for another teacher.

c) Phone Call Home. Students sometimes like this so their parents hook them up with a little sumpin sumpin.

d) Free Time.  15 minutes usually.

I have this little sheet that has a list of these things and the student can pick which one they want.  I put their name on it when they get a pass, and I also put the date and try to use colors and what not.