1) Have an offering which is extraordinarily difficult to hire for at the moment which...
2) ... accelerates growth of the startup in ...
3) ... a demonstrable manner as evidenced by...
4) ... past successes which are identifiably associated with your name in...
5) ... the minds of people who have authority to cut checks.
That's really the most helpful answer I can give you. "Learn SEO, A/B testing, AdWords, viral acquisition, etc etc." is also as true as the last 47 times I said it but less complete as an answer. There exist few reliable ways to hire for these at the moment and they are, at least potentially, all worth $$$$$$. However, putting out your shingle doesn't get you $x0,000 on your first week.
Does that rule out almost all consumer market startups, because they aren't making anything and don't seem terribly interested in doing so ("We're focusing on growth")?
I don't know what to tell you about how to value labor in companies that don't generate revenue. They all have some metric by which they justify spending hundreds of thousands of dollars a quarter on salaries; presumably, the same metric applies to contract work?
