MacBook Pro Slowing Down

I have a MacBook Pro that is about an year and a half old. The last couple weeks I've noticed that it's kinda beginning to slow down. Everything from connecting to a network or bluetooth device to opening a program, PDF or even finder is taking a hair longer that it should...just enough to be a little frustraiting. The only changes I've made recenty is I've installed Lion...also my hard drive is about 35gb from capacity because of some new apps added. Anyway, any thoughts? I really want my mac running in tip top shape.