For today just a few hints from various sources, namely how to make monsters more unique.
This is really advice gathered from a few different products so view this more as a list of tips than a burst of originality.
1: Don't tell the players the name of the monster, unless it's an in-setting name (the bull demons of the old temple, rather than minotaurs)
2: Change descriptions around (describing gnolls as hideous cavemen f.x.)
3: Swap abilities or add them (take a gorgon but make its breath poisonous instead of petrifying)
4: Make monsters unique and don't reuse them. If you have a group of them, they only belong to one villain or one location (lizardmen being a remnant race that evolution abandoned, the last tribe hiding in the Desolate Swamps)
5: Strictly avoid any type of "lore" abilities in the game. Actual, concrete information about monsters should always refer to one specific creature.
6: Avoid cannon-fodder monsters. If you have orcs, determine why they exist, where and how (even in the lord of the rings, orcs lurk only in certain, specific areas). Alternatively, swap them out for evil humans (you can keep the stats and even behaviours. Hobgoblins are militant invaders, while goblins are scavengers and raiders).
7: If the group does encounter a group of monsters (say, the wraiths that inhabit the tomb of Gazhkar), throw in a few "boss" types with more hit dice, different abilities or superior abilities.
What tricks have you used for monsters?