Here are a few things you need to think about:
1) How much do the founders need you? This will be helpful in negotiations.
2) How will your role in the company effect its success?
3) How important is salary to you?
All the above will give you an idea of how much equity you can get (or are worth). Use the answers to these questions to help you negotiate.
If salary is really important to you then expect less equity. If you really believe in the company and are willing to work to make it successful then maybe you should go towards more equity and take less salary. This will pay off (much more) in the long run.
I would recommend that you let them make you an offer first. This will give you a baseline on what their expectations are. They might say.. 60K/year and 20,000 options. You need to ask them about the options:
1) What percentage of the company is the employee options pool? This might be 10 or 20%.
2) How many shares are in the option pool, or...what percentage of the option pool is 20,000 options. This will help you determine what actual percentage of the company you may be entitled to.
3) What is the vesting schedule?
Remember one thing when negotiating: You need to ask or you will not receive :).
Good luck!