If the dealer has an upcard of 2 -9, double down.
Otherwise just hit.
You'll often receive a 10 when you take a hit (there are 16 of them in the deck, after all), and if you have a total of 10 to begin with, then your chances of winding up with a 20 after taking a hit is pretty good. A total of 20 is an excellent hand, since the only way the dealer can beat your 20 is if she gets a 21.
If the dealer is showing an ace or a ten though, there is a chance you'll either get a push or get beat by a dealer 21, so it doesn't make sense to get more money out there if the dealer has that ace or ten.