It's common for a shifter to stretch out a little after it's been used for a while. Where the gear shift cable go into the rear derailleur there is a little adjuster with nubs on it. It's called a barrel adjuster and you want to turn it counter-clock wise to make up for the cable stretch.
To start off turn it one full turn and see if the shifting is better. If it still slips hake another 1/2 turn. It may take several attempts before it's back to normal.
DO NOT adjust any of the screws on the derailleur.
http://bicycletutor.com/adjust-rear-derailleur/