I mentioned this in another answer (http://answers.onstartups.com/questions/1760/when-do-you-split-up-the-equity/1768#1768), but I'll repost a piece of it here:
At our first full-team meeting (I'd met my co-Founders individually, but not all three of us together), we came to an informal agreement on the percentage split. However, I wanted to make sure they would do their work, so we also came up with a vesting plan.
I wasn't worried about the Developer, so he got 10% to start, and 5% for each of three milestones. The third partner was the one who seemed like he might need some extra motivation, so we started him at zero, and gave him 5% for each of three milestones - each harder to attain than the previous one.
When I took this to our law firm during the incorporation process (S-Corp), it took some explaining, but eventually he was able to make the vesting work out.
That third partner is basically in sales and business development, and his three vesting milestones are all based on sales. So the short answer is "YES, it's possible." :)
The same caveat applies: By default, any shares that don't vest for the other partners revert to the company, which means they are split among the owners. If you want them to revert to YOU, make sure the lawyer puts in a clause that states that you have the right to purchase any unvested shares from the other partners, either at a certain time, or in the event that they leave the company. That way, you prevent your share from becoming diluted.