Now that we have a fair system set out, there is one important principle. You must have vesting. Preferably 4 or 5 years. Nobody earns their shares until they've stayed with the company for a year. A good vesting schedule is 25% in the first year, 2% each additional month. Otherwise your co-founder is going to quit after three weeks and show up, 7 years later, claiming he owns 25% of the company. It never makes sense to give anyone equity without vesting. This is an extremely common mistake and it's terrible when it happens. You have these companies where 3 cofounders have been working day and night for five years, and then you discover there's some jerk that quit after two weeks and he still thinks he owns 25% of the company for his two weeks of work.
I made this mistake and will never make it again.
[1] Earlier thread on Cruise where I defended the position that a founder deserved full equity (modulo later dilution) because his cofounder didn’t think to establish a vesting schedule or, failing that, to buy him out: https://news.ycombinator.com/item?id=11741652