In an FPS, I like to flank the enemy; be in a spot that I should not be able to be in, faster than they think possible.
Its hilarious to be standing behind an enemy's claymore, throwing knife in hand, standing over him, waiting for him to turn around.
In an RTS, I like to use my patented lure and nuke strategy, especially in SC1.
As protoss, you get a handful of Zealots to run into an enemy's base, kill something, then run out with the enemy hot on their heals. When playing against computers, you need some ranged units to keep them chasing you, so I have a couple dragoons waiting halfway between my zealots (runners) and reavers (nuke). Zealots run to the reavers without stopping, and the dragoons step up and hit the enemy a couple times. Then they run back as well, and my 5 reavers decimate whatever the enemy is chasing me with.
Against players, I'll modify this a bit by having the runners right straight out of their base, and the nukers meeting halfway (just imagine a T, where the bottom line is the nukers intercepting the chasers)
In an RPG, I like to have 3 unit types: Tank, Magic, I Kill You.
Tank is the guy with most defence and health, who will survive just about anything, and does pretty decent damage.
Magic is the guy/girl who does a ridiculous amount of damage to multiple targets, heals very well, or will assuredly kill 1 unit each turn.
I Kill You is the guy/girl who picks a target, and kills it. Usually in one turn, but sometimes in several turns (depending on health).
Every RPG I play has this kind of set up, modified a bit depending on party members and talent tree, but it has never failed me.
Flanking works in RTS' as well, but not as well as it does in FPS where the player can only look 1 direction at a time. Although it is hilarious to see an enemy try and run away, then get shelled by sieged tanks, or meet my invading force at his front door as reavers drop into his manufacturing core.