At the beginning of the year I hang small paper bags on a side bulletin board, one for each period, block, or subject. Each time I grade papers I put my initials in the upper corner of any paper receiving an A. If I'm giving a test, I may give initials to As, Bs, or Cs. This is just my way of saying great work. The students then put their names on top of my initials, tear the corner off, and drop this into the appropriate paper bag.
Once every month we hold a drawing. I usually draw out 3 names and have a box of prizes to select from. Prizes can be most anything but my students love the coupons that say they can bring a snack to eat on a particular day during a particular period or block. This reward system is an easy way for me to let the students know that they've done a good job, plus it's relatively inexpensive.