Hi Kim,
There could be a few things that are causing the slipping. Stretched chain, worn chainring teeth or just stretched cables.
For starters, try turning the barrel a few clicks downward. The barrel is located just to the left of the shifter and kind of under it. If I'm wrong, I sure hope someone tells me so! That has cured my sloppy shifting issues on a couple of occasions.
Do you have Zinn and the Art of Road/Mountain Bike Maintenance? That's a fantastic book with easy to follow diagrams and directions. Sheldonbrown.com is also a great place to learn from.
Good Luck!
Kristina