If you play a double of the opponents notrump as penalty, you may have the agreement to play "systems on" over your side's double. That is, if it goes 1NT-X-Pass, then 2C is Stayman, 2D is a transfer etc.

A better agreement is to play "over notrump on". That is the bids by partner of the doubler show the same thing that they would directly over a notrump opening, but deny enough useful points to pass the double.

Say the partnership is playing Cappelletti (see: http://en.wikipedia.org/wiki/Cappelletti ) over notrump, and it goes 1NT-X-Pass or 1NT-X-XX. Now:

2C: Any long suit, asks doubler to bid 2D unless special hand.
2D: Both majors.
2H: Hs and a minor
2S: Ss and a minor
2NT: Both minors

If it goes 1NT-X-suit bid, play double by either partner is takeout, but passable by partner with length in the suit doubled. This "negative double" will help find fits as well as catch the opponents when they are in a misfit.

As to Maestro ( http://www.bridgematters.com/maestro.htm ), over a weak notrump, double only with 12+ points (instead of the 10+), to allow partner to pass with less (8 or more) and put the opponents in escape mode, where they might be nailed if they cannot locate a good spot to play.