Freelance contract gives client total code control. Need advice.

I agreed to write software for a startup last week. Their contract says they own all code I produce, even common bits I use often. One side says this is fine to keep clients happy and secure. The other side worries I lose the ability to work on similar stuff later. I really like this project but don't want to sign away my future work. What do you all think is a fair middle ground? How have you changed these clauses before?