I'm surprised no one mentioned this so far: check to make sure the fins on your outside unit are clean.

My parents had this issue when I was visiting. I went outside and looked at their unit and it looked like it was covered in carpet. I took a hose and gently rinsed off all of the dirt and pollen and they were back in action.

Others already covered a lot of good points: make sure it's not froze (it likely is if you've been running for a while and it's not keeping up) and change the filter.

If that doesn't work you likely need refrigerant.