This is, by far, the most popular practice drill with my piano students. If you are looking for new ideas for how to practice, try this Smarties Drill. First, you start with a number of Smarties (between three and five is ideal). It actually doesn't need to be Smarties. You can use carrots, raisins, M&M's, nuts or even marbles. Using something edible makes it more fun. However, try to keep it to snacks that are safe around your instrument (chocolate chip cookies or brownies would really mess up your musical instrument). Group your Smarties on one side of a Kleenex or plate:
Pick a spot in a song that requires extra practice. Maybe it's a spot where you consistently have a few few notes or where the rhythm is just a little wonky. Play through the spot slowly.
Each time you play through that spot cleanly, move one of the Smarties to the right-hand side of your Kleenex (the "Success!" side):
Say that your first two run-throughs were pretty good, but your third wasn't so hot. You have to move one of the Smarties from the "Success" pile to the "Oops, try again" pile.
Once you've done enough clean run-throughs to get all the Smarties to the "Success!" side, you can pat yourself on the back and enjoy your Smarties.
Don't forget to work the trouble spot that you worked on back into your piece in stages. Add the bar before, then the bar after the trouble spot. Keep extending your practice area until you can play through the entire section pretty well. That's a different practice drill.