Be prepared to hold domains for years. I have been at this since 2004, owned about 200 names over the years, still own about 40, sell about 20 or 30 a year, usually for not much, but sometimes I get lucky. I do this part time. At the end of the year, tax time, I usually end up where I spent $500 on domains and hosting, and made about $5000 or $6000 on domain sales ($2000), Adsense ($1500), link sales ($1000), affiliate programs ($1500).
Sometimes you can flip them right away, but mostly it doesn't work out that way, and sometimes you just have to let them go for cheap. Domaining is a lot of work. You have to really study. Study domain auctions and learn how to define a valuable name (EMD's, TLD's, LLL's, PR, DA, PA, age, traffic, content, amount of searches), and how to verify that stuff, learn what makes a domain name valuable, the best places to buy cheap, and the best places to sell, how to market to end-users, how to market to other domain investors, how to build a money-making website- even if it just makes $10 a year in the meantime so you don't have to spend money on reg-fees, start writing articles about things you know, get some Adsense on there, get your link on some other sites- make your domain worth more.
Most of the time, I'll spend $1 to $25 for a name I think I can get $100 to $10,000 for. But I only get $30 or $50. Then, I'll sell one I thought was worth $50 for $350. You just never know sometimes. The more you hang in there and keep trying different things, the better you will get at it.