Have you upgraded your phone to 2.2 (Froyo)? The software for Windows is available from HTC's website, but I believe if you have a Mac you need to take it to a T-Mobile store to get it updated.
Froyo allows you to install some apps to the microSD card, which saves a lot of space. You may consider buying a new microSD card if you find that along with your pictures and music, your apps take up too much space. The Slide comes with an 8GB microSD card, and can support up to 32GB cards. I've personally used 16GB and 32GB cards with no problem.
You can limit the amount of space the browser is allowed to use to begin with in the settings menu; that way you don't constantly have to go in and clear everything out.
Before I updated I found that unless I had 17-20MB of internal storage free my phone would crash, especially during calls all the time, to the point of it being unusable.
Hope that helps.