Are the opponents vulnerable? Get that double card ready. Be less inclined to double non-vulnerable opponents; a 1 trick set will turn 50 into 100, not enough of a reward to take the risk.

Control the trump suit: look for length and strength in the opponents suit (preferrably behind the bidder).

Shortness in partner's suit(s): this increases the likelihood of partner's high cards taking tricks as well as decreasing your side's chances of making a game/slam that might make the penalty insufficient.

Here is a hand from the Dallas Nationals where Sue/I took advantage of our overbidding opponents.

With both sides vulnerable, I held: 932 QJT2 Q9 7432

Not a very exciting collection at first glance. Sue opened 1 and RHO overcalled 2. Normally I am happy to raise partner when I have a fit but this hand is defensive in nature. I rate to win 2 tricks in hearts on defense, but those will be of little use on offense. I have little distribution, bad trumps, and few values so I passed and awaited developments. I had not decided yet what to do if Sue doubled in balancing seat (playing negative doubles, a re-opening double by opener is for takeout giving responder a chance to pass for penalties). Sue did re-open, but with 2. I had no where to go and was set to table my hand, but LHO couldn't leave well enough alone, balancing with 3. This was a contract I felt comfortable doubling, having at least 2 sure tricks opposite a partner who was able to bid twice. There are no guarantees; there could be very wild distribution out there, though the bidding suggests otherwise. A great player once said, "If the opponents do not make any doubled contracts against you, you are not doubling enough." Two of the factors listed above are in our favor, we have good trumps and the opponents are vulnerable. The spade fit is a slight negative, but the opponents are high enough where it should not be a problem. Also note that we have "slow" heart tricks (Qs and Js) which are of no use on offense, but worth tricks on defense. Now we need to take as many tricks as we can to inflict maximum punishment. No reason to do anything clever, lead a low spade. Dummy comes down and it looks promising (for us):

T5 K98 6532 K865

The play begins:

2, 5, A, J
7, K, 9, T

The play to the 2nd trick is subtle; if partner is watching closely they know we have the 3 and 9 remaining. The 9 should signal to partner that we have something in diamonds (the "higher" ranking side suit). This may or may not make a difference, but it can't cost. The play continues:

J, 7, 5, Q
This is going to be bloody. No reason to falsecard on this hand, we play a high club (standard count) to show an even number.

4, 7, 9, 2
Q, 3, T, A
T, 2, 6, A
K, 8, 3, 5
9, 8, 4, K

This is a key play. One of the dangers of holding long trumps is that sometimes we can be end played. Partner has done very well, and we have been careful to keep our 3rd spade (a card that declarer cannot remove) as an exit card. Declarer can do nothing from here and must lose two heart tricks.

Consider what would happen if we had thrown our 3rd spade (or partner had led 1) rather than a club. Declarer would ruff a club in his hand and lead a low heart towards dummy. We must play the T (or he will play the 8) and declarer wins the K. He then lead the 9 (or 8) and plays low from his hand. We are now end played and will lose one of our heart tricks.

Surprise, surprise; a hand that showed little promise has turned into a surprising +800. Aggressive doubling and accurate defense will make the opponents think twice about overcalling.