A friend of mine was in the same situation, and I compiled some articles for him. His situation had to do with negotiating equity, so keep that in mind. Here they are:
http://mba-mondays.pandamian.com/tableofcontents/
Read from "Employee Equity" through "Employee Equity: How Much?"
Also: http://www.payne.org/index.php/Startup_Equity_For_Employees
An interesting perspective on on mathematical model of calculating how much equity you "are worth": http://www.paulgraham.com/equity.html
The number that you get from Paul Graham's calculation tends to be higher than what you usually end up getting.
A blog post on the process that someone went through in negotiating salary/equity: http://keen.io/blog/29904565692/how-i-negotiated-my-startup-...
This is entirely unrelated to negotiating equity, but is useful to know about (this only applies if you're living in the US): If/when you do get stock/options your employers should inform you about an 83(b) election. This is a tax form that allows you to save on taxes due to stock grants being treated (at time of vesting) as income. Read about it here: http://www.fairmark.com/execcomp/sec83b.htm. They'll probably tell you about this and give you the forms for this if/when you get stock.
Concepts you should have a good handle on after reading those articles: preferred stock (liquidation preference), common stock, dilution (and how it occurs), stock options vs. giving stock, vesting (cliff). If you don't, do more reading!
Above all else, remember that if your startup is pre-revenue, valuations are extremely tricky. Even the valuation of a pre-revenue startup as set by a funding round can be unreliable, since it's essentially a made-up number (made-up by VCs/angel investors). Consider that if even VC's and angel investors barely know what they're doing, imagine how hard it is for employees to properly place a value on their equity. A lot of it comes down to how confident you are in the founders/management of the startup (which is often how investors approach this problem). Hopefully you have worked with the people long enough to get a good handle on how "determined" and "resourceful" they are.
If you have any more questions, see my profile and send me an email (replace the "|" with an @)