21- Trump Card Tactics:
Exchange: This is one of the most useful cards in the game. You can steal cards you need, foist bad cards off on your opponent, and generally control the state of play. It also can't be cancelled or countered, and it works even better against an opponent using Perfect Draw, as this tells you exactly what exchanges will work.
Two-Up: Don't think of these as simply increasing points. Think of them as allowing you to win the next two or four or ten hands in a single stroke. The best part is that you get to draw another card while doing so, which allows for the possibility of even more Two-Ups to play. Just be careful when using them against the higher level opponents in Survival+. Lay out all your Two-Ups at once, and you're just asking for a Destroy+ to sweep them away.
Destroy: Destroys the last trump played by your opponent, that is still on the board (does not effect trumps like Oblivion, which are immediately discarded). Useful in many circumstances, but you should really try to save these for the boss fights, when they are often the only card that can help you. When upgraded, it becomes a monster that take out an opponent's whole game plan, but it's still worth saving for boss fights.
Remove & Return: Useful in their own right, but try pairing them with Exchange for real tactical value
Go for 17, Go for 24, Go for 28: The higher values are handy if you've gone bust, or are facing the Halfhead's Curse trumps. Go 17 is useful if it's too risky to draw another card, but it's superuseful against enemies playing Perfect Draws- turning a 21 or 20 into a loss.
Perfect Draw: Handy, but use sparingly. Playing this card allows your opponent to deduce the exact value of your hidden card, and respond accordingly. At the same time, you can learn a bit about your opponent's hand by which card you do or don't get.
Shield: Given the firepower some of enemies throw around, these are not always as useful as one might hope, but they can save you from harm on the lower levels, when you don't want to spend a Perfect Draw or other trumps to salvage a hand gone bad.
+[Value] Cards: Not as silver-bullety as a Pefect draw, but they can certainyl put you where you need to be. Can also be used to confirm hunches about your opponent's hand (particularly in conjunction with Return), and can be part of a moving-the-goalposts gambit along with one of the Go For trumps.
Trump Switch: A judgment call- more cards is always good, but maybe you have good stuff in your hand. Worth saving for a really bad jam, when you need a completely different approach to prevail.