One of the best articles I've every read was from Joel Spolsly on Stack Exchange. The big thing to remember is the split based on risk and vesting period!
I can't find the original post but It was reprinted on Business Insider (which I F'n hate) http://www.businessinsider.com/how-to-allocate-ownership-fairly-when-forming-a-new-software-startup-2011-4?op=1
Part 1: -- Cut'n paste for those that don't want to visit BI ----
Where Twitter And Facebook Went Wrong --- A Fair Way To Divide Up Ownership Of Any New Company
This is such a common question here and elsewhere that I will attempt to write the world's most canonical answer to this question. Hopefully in the future when someone on answers.onstartups asks how to split up the ownership of their new company, you can simply point to this answer.
The most important principle: boldFairness, and the perception of fairness, is much more valuable than owning a large stake. boldAlmost everything that can go wrong in a startup will go wrong, and one of the biggest things that can go wrong is huge, angry, shouting matches between the founders as to who worked harder, who owns more, whose idea was it anyway, etc.
That is why I would always rather split a new company 50-50 with a friend than insist on owning 60% because "it was my idea," or because "I was more experienced" or anything else. Why? Because if I split the company 60-40, the company is going to fail when we argue ourselves to death. And if you just say, "to heck with it, we can NEVER figure out what the correct split is, so let's just be pals and go 50-50," you'll stay friends and the company will survive.
Thus, I present you with boldJoel's Totally Fair Method to Divide Up The Ownership of Any Startup.bold
For simplicity sake, I'm going to start by assuming that you are not going to raise venture capital and you are not going to have outside investors. Later, I'll explain how to deal with venture capital, but for now assume no investors.
Also for simplicity sake, let's temporarily assume that the founders all quit their jobs and start working on the new company full time at the same time. Later, I'll explain how to deal with founders who do not start at the same time.
Here's the principle. As your company grows, you tend to add people in "layers".
item 1 The top layer is the first founder or founders. There may be 1, 2, 3, or more of you, but you all start working about the same time, and you all take the same risk... quitting your jobs to go work for a new and unproven company.* item 1
item 2 The second layer is the first real employees. By the time you hire this layer, you've got cash coming in from somewhere (investors or customers--doesn't matter). These people didn't take as much risk because they got a salary from day one, and honestly, they didn't start the company, they joined it as a job.* item 2
item 3 The third layer are later employees. By the time they joined the company, it was going pretty well.* item 3
For many companies, each "layer" will be approximately one year long. By the time your company is big enough to sell to Google or go public or whatever, you probably have about 6 layers: the founders and roughly five layers of employees. Each successive layer is larger. There might be two founders, five early employees in layer 2, 25 employees in layer 3, and 200 employees in layer 4. boldThe later layers took less risk.bold