I am being allotted some equity in a startup. What can be some reasonable clauses which can help me force buy-back or impose selling preference later?

I had been working as a developer in a startup and getting paid through equity. My work is for a fixed amount of time and I can leave after that. I want to make sure that the equity given to me can be en-cashed at some future funding round. Without any sort of preference clause, the funding rounds can just get used in paying back the previous investors and I might not be able to sell my equity soon.
Can anyone suggest some clauses which can solve my purpose and are not very problematic to the company or it's present and future investors?