http://www.minmaxboards.com/index.php?topic=578.0Have a look in there, it's a thread about increasing monsters' difficulty without increasing their CR... should be along the lines of what you want.
But you'll have to put in a little work all the same. Ready-to-use optimized monsters/NPCs will not necessarily challenge the specific setup of your group. For example, if the PCs are simply a set of very able bruisers, a single equal level caster can likely shut down large parts of the group, so his summons can mop up the one or two that evade his BC spells. If, on the other hand, your PCs include focused battlefield controllers, you need different tactics - maybe flying, ranged-capable, incorporeal monsters or something?
Also, think about adding a different sort of difficulty to your encounters. For example, my group just had to stomp some seriously underwhelming Gnolls yesterday evening. The problem was, they had hostages, bound to poles, drenched in flammable oil, and standing next to a fireplace. Another hostage was suspended over a rotating sawblade. Well, we won the fight in the end, but our 'score' wasn't perfect for all of that... nuff said.
Hostages, fragile and valuable goods standing around everywhere, a source of "everything blows up" near the battleground, an uncontrolled demon in a calling circle in the middle of the room, a sleeping dragon next door - all those and more can be put to use in order to make fights more challenging without increasing the power of the opposition.
