These "12 Rules of Domaining" started off as a group think thread at Discuss Names and aren't true in every situation but will serve you well most of the time. If you would, please help me add to this list with suggestions of more rules if you think one of your important domaining rules is missing:
1) In general, don't register names rightaway; think for a while, then if you still feel it's worth; go for it.
2) Don't register anything that has more than three words
3) Don't register every TLD for your brilliant idea - grab the .com and see how it does
4) Don't register 1 billion domains in your first month - take some time to learn from the 999,999,999 mistakes you made
5) Do assume that you will never develop the domain - let the name carry its weight
6) We all over-evaluate our own domains because we have an emotional investment in them. Remember this so you won't miss out on good offers.
7) Do market your domains, even if they are weak.
8) Do listen to those who have made the mistakes you are making
9) Do learn to "develop a site" - sometimes names can't carry their weight
10) Do examine "typo domains" to see if they are REALLY typos (letters need be close on the keyboard for example)
11) Don't substitute numbers for letters that have a visual resemblance
12) DON'T: use a credit card for registering domains. Use your regluar debit card and watch your balance shrink. Learn that domains cost a lot of money You don't want to kick off this domaining business with a huge credit card debt.