From 20+ years of practicing law, 15 as a self-employed practicioner, someone who has also been developing a web enterprise for 5 years:
1. Know your reputation and if it is known by the customer. If know it they are prepared to pay, but are they able? Decide how much quasi-pro-bono work you are prepared to do for a good person/cause. Sometimes allow good paying work/clients to indirectly underwrite your good works.
2. Know the market, i.e., your competitors. Some of what you offer is reproducible. Some - creativity, brilliance under pressure, connections, etc. - is not. If you are known for the uncommon qualities people will be prepared to pay. Always ask "where did you learn about me?" It can be illuminating.
3. Know your overhead, what you have to make hourly to meet your weekly/monthly overhead. Profit only occurs after meeting overhead. (Of course, I once advised at WW that when starting up an business the cardinal rule is KEEP YOUR OVERHEAD DOWN)
4. Know yourself and trust you instincts. If your skin bristles when you meet the prospect don't get involved. If you take the job and things start going wrong you will hate yourself for second guessing yourself. Don't do it. Ever. No exceptions. Right now. For this one the rule applies? (Get it?)
Client's that present themselves - obliquely or otherwise - as knowing more than you (in a sense) are best advised to represent themselves, that is, to do their own SEM/SEO/Design. If they don't promptly humble themselves (acknowledge their lack of expertise) when you point out who the expert is - dump them 90% of the time. Only really bright people should be suffered when they present that "they know something". Really bright people also get it: that's there's other important factors than just IQ. I mean the brightest. People out to prove they are smart - dump them instantly. Really bright people have nothing to prove. At worst they are just curious or eager to help and your job is to give such people direction so their energy and interest is well invested.
5. Know the aggravation level of the work. Bill accordingly or you will be unhappy, and find yourself putting off doing the work.
6. Know that if you take job X for Y dollars/hour that you will have issues and problems when job A comes along that is able to pay Yx2 dollars/hour. The most important decision is often the work NOT to take. Refer the work to a lesser able/qualified acquaintance - subtly expressed - and you may make 2 people happy. You will also likely lose the client forever. C'est la vie. Do not refer jerks to friends. Refer jerks to .... well, you get the idea. (They will likely return the favor.)
7. People who flinch/gripe about money in the beginning won't be any happier as you move along and problems, not of your making, come up. Make your retainer suitably high - appropriate to the work - because if they can't pay the retainer they won't be able to pay the future bills. No kidding.
I'm writing this on the fly and likely will come back to see what I've missed. Just thoughts of the moment, prompted by reading the 60 reply thread about billing.