Buidling on what BCI said...

Along with tires, for the girls who have f/s light bikes, if you feel like you're bouncing all over, make sure you have your shocks set up properly. If you never bottom out your shock, then you need to make an adjustment! We've had women come into the store with their bikes for a tuneup and I couldn't even compress the fork and rear shock. These were girls who were about 5 inches shorter than me and light weight. There is no way they were getting the benefit of full suspension.

Some bike shops don't take the time to teach you how to properly set up your bike. There should be a little sag in the suspension just sitting on it. After that, take it out for a few rides with a marker on your shock to see how much travel you get. If you don't get full travel during an aggressive ride, adjust it. You don't want to bottom out all the time, but you should be on occsaion. The suspension is definitely something you need to play around with to get it set to your preference. Take a pump to the trail and play with the settings.