Double down if the dealer has a 2 through 10 for her upcard.
Otherwise just take a hit.
There are 16 tens in the deck, which gives you an excellent chance of making a 21, which is an unbeatable hand. (The worst you can do is a tie.) It's also impossible for you to bust in this situation. By doubling down in this situation, you're taking advantage of the great odds in your favor by getting more money into action.
If the dealer has an ace showing, there is a good chance that the dealer has a 21, so there's not much point in getting the additional money out there.