I dropped out, too (it was to a programming job). This was quite some time ago (1982).
I tried to finish school later, but work always won. I never had the discipline to do both.
To keep up:
1. Read CS textbooks and do small projects to make sure you understand the material at a practical level.
2. Join the ACM and keep up with interesting publications from their electronic library. (The Usenix papers are free, but more limited in scope).
3. Learn a new programming language, server technology or OS environment at least every year. Keep your level of exposure to new things high and don't get too comfortable in one thing. Don't be a zealot and don't get stuck.
4. Read a LOT of code. (I read through the Unix sources, the source for TeX and a bunch of other largish packages just because they were there).
More recent things that I didn't have access to: Publically available course videos and material. My guess is that these are as good as the amount of time you put into them.