hello Matt

Is your host complaining about your site using up to many resources (CPU time/space)?
Is your bandwidth alottment no longer sufficient to sustain your site?
If your site is begining touse a lot of CPU time on the shared server you're on, or it is starting to feel really sluggish, then chances are you need to move on up.
If it's just space or bandwidth, depending on the price, just move up to a larger package. Sometimes it's almost cheaper to go with a VPS than to stay with a shared hosting package with a lot of space/bandwidth. Next you have to ask yourself "Do I know how to manage a server?"