Source: Defensive Trump Tactics By Richard Pavlicek
Besides the obvious case of ruffing declarer’s winner, the trump suit offers many opportunities for skillful defenders to gain a trick. This is not simply a case of winning a trick every time you have a chance — good defense requires a firm understanding of the available tactics and patience.
This lesson explains some of the finer points such as when to win your trump, when not to ruff, and how to promote trump tricks.
When To Win the Ace
The ace of trumps is a precious card; the player who holds it is able to control the number of rounds of trumps led. As a defender it is often an advantage to hold up the ace of trumps on the first round in order to retain this controlling ability.
[box]Generally, win the ace of trumps when you have one low trump left; i.e., with A-x win the first; with A-x-x, the second, etc.[/box]
Declarer wins the 
A and leads a spade to his king. West should duck. Declarer is now stymied. If he leads a spade, West can win and lead a third trump to prevent a second heart ruff in dummy; if declarer instead tries to ruff hearts immediately, East will overruff on the fourth round.
If West won the first trump lead, declarer could draw exactly two rounds of trumps before ruffing his heart losers.
The Forcing Defense
An effective defensive strategy is often to force declarer to ruff. The objective is to make declarer waste his trumps by ruffing; then he may not be able to draw the outstanding trumps, after which the hand will often collapse for him.
The foundation of the forcing defense is to lead declarer’s shortest suit, which logically will be your longest suit. Don’t be hesitant about starting this attack on the opening lead, especially if you have length in the trump suit.
[box]A forcing defense is most desirable when one defender has four trumps, most of which are low cards; e.g., A-x-x-x, K-x-x-x or x-x-x-x.[/box]
West’s trump holding is the reason for the aggressive spade lead; he hopes to start a forcing defense and spades is probably declarer’s shortest suit. East wins the ace and returns the two; 10, jack. West should next lead the king to force declarer to use one of dummy’s trumps.
Declarer leads a heart to his king, and West should duck. West plans to lead another spade, but he wants to force declarer to ruff in his own hand; hence he must not win the ace while dummy still has a trump. Declarer leads the Q and West ducks again. If declarer leads another trump, West will win the ace and lead a spade; if declarer instead cashes his winners, West will get a diamond ruff.
Don’t Ruff a Loser
Eagerness to win tricks can sometimes lose tricks. A common mistake on defense is to ruff a low card as second hand early in the play. This rarely gains and often costs a trick because the defender gives up one of his trumps while declarer parts with a loser.
Play tough, and wait for your ruff.
[box]The possession of three or four trumps is a burden on declarer, even when your holding seems worthless. Ruffing a loser will seldom help your cause.[/box]
West leads his singleton to try for a ruff, but the play goes unexpectedly. Declarer wins the 
A and leads the 2 toward dummy. If West ruffs, declarer will play low from dummy and save the king for later. Note the loss: West gives up his third trump, a menacing card, while declarer gives up only a low diamond. Is this a fair exchange? Hardly. West should discard a club on the second diamond lead and save his trump for later.
This deal is worth a careful study. Declarer cannot succeed on his own merits unless West makes the mistake of ruffing early. Note that if declarer ever ruffs a club in dummy, West’s 
10 will be promoted into a natural trump trick.
Ruffs and Overruffs
It is sound defensive strategy to try for tricks by ruffing, especially when you or partner are in a position to overruff declarer or dummy; but the tendency is to try for these ruffs too soon. Declarer may be able to discard a loser, thus nullifying the effect of the ruff. Or even worse, declarer may gain a trick outright when a defender has to ruff with a natural trump trick.
Accurate defense usually requires careful timing. Consider the alternatives before you routinely try for a ruff or an overruff.
[box]Before leading a loser for partner to overruff declarer, try to collect your tricks in the remaining side suits.[/box]
East knows West’s lead is a singleton or doubleton, but he must resist the urge to play three rounds of diamonds immediately. Even if West had a singleton, it is unlikely that an early ruff would gain as it would set up a diamond trick in dummy. After winning the K, East’s best strategy is to shift to the 4.
West wins the A and leads a diamond (better than a spade in case South held the K) to East’s ace. East next should cash the K (key play) before leading a diamond. This allows West to score a trump trick regardless of declarer’s play.
Note that if the defenders failed to cash two spade tricks, declarer would simply discard a spade on the third diamond.
Refusing To Overruff
In many situations a defender may gain by not overruffing when given the chance. This might promote an additional defensive trump trick because declarer was forced to waste an important card on the trick the defender declined.
Even if refusing to overruff does not promote a trick, it sometimes is effective in deceiving declarer as to the lie of the trump suit. He may assume you don’t have a particular card and misplay the hand later on. At the very least it will cause some confusion for declarer.
[box]If the card you would overruff with will always win a trick, you should usually discard unless it is urgent to obtain the lead.[/box]
West leads the king in his partner’s bid suit then continues with the nine, which East overtakes with the 10. East leads the A and South ruffs with the Q (if South ruffed low West would overruff with the nine). West should refuse to overruff and discard a club or a diamond.
Assume declarer crosses to dummy with a club and tries the spade finesse losing to the king. West now leads a diamond to East’s ace, then a fourth heart completes the trump promotion. South must ruff high to shut out the nine, and West is assured of another trump trick.
Note that each time South ruffed high, West’s 9 was promoted another notch until it finally became a natural winner.The Uppercut
Another kind of trump promotion occurs when one defender ruffs — usually with his highest trump — to force declarer to overruff with a higher trump. This is known as an “uppercut.” The goal is to weaken declarer’s trump holding in order to promote an additional trick for the defender’s partner.
The uppercut is most attractive when your trump length is equal to or shorter than partner’s and you have a medium-sized card. Trump holdings like 9-x, 10-x, J-x or Q-x would be ideal.
[box]If your own trump holding cannot win a trick, be alert for an opportunity to ruff in front of declarer or dummy.[/box]
Declarer wins the A and leads the K. West sees the uppercut possibility so he grabs the A — if he followed the general technique of winning the second round, East would be out of trumps.
West cashes the Q then leads another heart which East ruffs with the 10. If declarer overruffs, West’s nine will be promoted into a trick; if declarer discards, East wins the trick and cashes the A.
There is some controversy as to which heart West should lead for his partner to ruff. Some players would lead the 4 to force the uppercut. The correct technique is to lead the 10 and trust East to uppercut whenever there is a chance to gain — East might not have a second trump.