Double down if the dealer is holding a 3, 4, 5 or 6.
Otherwise take a hit.
If the dealer's upcard is a 3, 4, 5, or 6, then she probably has a stiff hand, which means she has a good chance of going bust.
The highest card you can be dealt when you have a soft 17 is a ten, which gives you a new total of 17, which will beat a lot of dealer hands.
These two factors combined make doubling down a good way to get more money in action while you have an advantage.