Say your startup:
Hourly rates seem a simple enough solution, but as I learned when asking another question can often be seen as too complicated and incurring a lot of overhead. Milestones are another seemingly clear choice, except if there's one thing I've learned over the past six months it's that estimates of the effort required for a particular feature or milestone is hard, very hard.
So, what are some of the best practices for 'fairly' sharing profits before investment in a small startup with a highly dynamic team?
I think I finally see where you're coming from, after the little discussion we had in the comments of the question.
You've found a way to divide up ownership of the company (equally, or otherwise) but you're at a point where you want to start drawing salary for your work up front, since you're bringing in some money.
The real solution is, do whatever makes your team happy. That's all that really matters. Just because some random person on a Q&A site said you should do something doesn't mean you should. It really doesn't matter what the "general rule" says if it doesn't make your team happy.
After thinking about this for a while, I think if I were in your situation, I would basically pay in IOUs. Figure out how much you think everyone is worth, at an hourly rate (perhaps everyone is the same) and keep track of the hours you put in. When the company is bringing in money (through sales or outside funding) you pay off the IOUs. If you do this, everyone knows that all of the hours they work will eventually get paid off, when the business becomes profitable. And, it doesn't mess with shares and ownership of the company.
In the mean time, you're bringing in some money, and some of the people want to grab a part of that, so they don't need to live off of just Ramen. With the money you get, you can start to pay off some of the IOUs to whoever needs it. The trick here is coming up with a system that seems fair to everyone.
I might suggest that the people who are only doing this in their evenings and weekends, and still hanging on to their full time job, should probably should offer to not take any of this money up front, or at least, not very much of it. They should wait until the big bucks are coming in, and they move away from their day job. They'll still get paid for their hours, just not until later.
If that doesn't fully cover it, I'd probably suggest you pay off the earliest hours/IOUs first. If in month #1, you work 160 hours and the other two work 20, you would take the money you bring in and pay off the IOUs for your 160 hours and the others' 20 hours before moving on to month #2. If it takes four months to finish off month #1, so be it.