1. When buying or regging a domain, have a plan from the beginning for how you are going to make money from the name: outbound to end user, inbound by listing on specific platform, by auction, parking, etc.
2. Before you buy give yourself a budget and stick to it. Don't get caught in "auction fever".
3. Keep good and honest records so you know when, if, and how much you are making or losing.
4. You will make mistakes. Don't hold on to a bad investment; especially don't keep renewing a loser. Dump it for nickels or let it go.
5. Learn how to look for drops and how to buy at auction, closet, etc. of registrars, especially GD which is where 80% of domainers' names are regged.
6. Find a buddy or mentor and communicate with them, share ideas, bounce your questions off of them, learn from their experience.
7. You never go broke by taking a profit. Don't cry over money you leave on the table. You don't own that domain anymore; learn from it and move on.
8. Don't try to hit home runs. Single are good.
9. Be wary of fads, but don't be afraid to jump on board... just carefully. The trend is your friend, but too much trend is your enemy.
10. Especially in the beginning, find a niche and focus on it; learn what you can. There is money to be made in brandables, 4L's, 5L's, keywords, numerics, new TLD's, etc.... but you don't have the bandwidth to learn them all.