Joel Spolsky has a wonderful mantra on equity:
Fairness, and the perception of fairness, is much more valuable than owning a large stake.
Joel's equity dividing answer to this question offers what we feel is a comprehensive answer to this nagging equity question.
If you find yourself wanting to ask an equity dividing question, please read Joel's answer first before posing the question. It's chock full of useful information and has a wonderful example to guide you through the process.
Additional Equity Questions and Answers
If Joel's answer does not answer your equity dividing questions, then take a look at a couple more questions that dive into different aspects of equity, founders and working for stock.
Lastly, looking through the questions tagged 'Equity' is often very helpful.
If you want a handy equity calculator, one of our moderators, Alain, has one. You can find it here
Feel free to add any additional links to questions or answers that expand on the topic of equity.