Erratic K said:
For ways to challenge high level characters check piratecat's story hour:
http://www.enworld.org/forums/showthread.php?t=779&page=1&pp=20
My personal advice is:
1) mix the encounters, give them some easy encounters and then give them some tough. Cake encounters allow the characters to use all their cool powers, but set up expectations so you can surprise them later. If you are having problems with tough, go up CR. I find at higher levels advancing monsters doesn't always cut it.
2) make consequences for defeating foes or pursuing tactics... Piratecat's thread above is like a giant reference tome for that.
3) have fun... the point is for everyone to have fun. If you aren't, or your players aren't examine why and find something fun todo.
-E
I'd have to agree with EK here. *starts singing* You've got'ta, got'ta, got'ta mix it uuuuup!!!

Seriously though, every encounter isn't supposed to be a gritty, bare-fisted round with the unholy angel of death. I think I read somewhere actually that the average encounter is supposed to expend around 20-25% of the party's available resources (i.e. spells, hp, item charges, potions, special daily abilities, etc...). If every encounter was a harrowing brush with oblivion that would scar any sane person for an elf's lifetime, then not only would there be a serious shortage of adventurers, but those type of encounters would lose some of their tension and suspense and suprise and whatever else. Which would be cooler from a DM's point of view to you? A: The party pauses to catch it's collective breath after slaying ascore of trolls in some cavern. Not ten seconds later a pair of fire giants walk around the corner, spot the party, and charge. The PCs sigh, settle into fighting stances and prepare for the worst.... again. Now B: After finally slaying the last of a troll host the size of which the characters have never seen nor heard the like of before, they groan and sit down to lick their wounds. Before they can ponder what caused such a large congregation of the beasts they hear faint tremors in the ground, looking up they see a massive form turn lope into view from around the corner. Craning their bruised necks they see a massive orange-skinned humanoid wearing armor as thick as steel doors and a brace of troll heads tied to it's belt. The creature stares for a moment at the battered and haggard party and then grins wickedly, drawing a sword the size of a small tree as it advances menacingly. And then the second one appears behind it... The PCs, who are used to near-effortlessly vanquishing hordes of petty undead or rogue beasts or what-have-you, feel their blood run cold as they seriously wonder if they'll ever see the light of day again...
So yeah. Sure, if it happens all the time they'll always be pretty worried about their characters' continued, well, life, but this way you'll probably get a few to crap their pants or something

Ahhhh, the joys of being a DM
Anywho, whatever you do, always remember the golden rule (as was mentioned earlier): Make sure you and your victims are having fun!!
