If you've never plugged your phone into your computer and seen what you've got on it's hard drive, I encourage you to do so. It's amazing what your phone holds onto without you even realizing it! I currently have an HTC Android that my mom gave me when my Blackberry crashed. Here's what I just discovered...
Think that you've deleted a photo you may have taken but then decided to get rid of? You deleted it from your photos folder via your phone, right? Well, go in and look at your phone's hard drive, and you'll probably find a totally obscure folder somewhere in there that's holding onto thumbnails or in some cases, larger sized images, of those which you thought you deleted. I had almost 300 thumbnails and larger images stored on my phone that I thought I had gotten rid of ages ago.
Also, I found a folder dedicated to screenshots that it took all on its own accord of web search pages that I'd been to by using my phone's "quick search google" feature. I mean seriously... do I really need my phone to be taking snapshots of google's homepage screen?
The other thing I got rid of was a folder of random mp3's that I know I didn't install on there. They came with the system, they're not necessary, they're not part of the phone's "sound options" for ringtones. They were unnecessary. And mp3's take up a lot of space.
My phone malfunctions from time to time, saying its internal memory is too low, but I couldn't figure out why. I onlypurposefully keep about 60 pics on there at any given time, I don't have music on there, and any little video clips are few and far between and they get transferred to my laptop and then deleted from the phone. But now I know.. it's no wonder! It's been keeping all these images and unnecessary files since the day I got it!
Go in, clean up your phone's files, and get back to a quicker, more responsive, fully functioning phone again.