Well, I suppose if you want to be a stickler, anything you do to toughen up a monster technically raises its CR. Of course, you're the DM, so you can choose to beef up the enemy and not give extra XP out.
I sometimes use one-shot or low-charge magic items. Give them a wand of fireballs that has only 2 charges left (in case the PCs should luck out and get it). Your idea of potions of heroism are good.
If your players have high armor classes, perhaps you should do things to reduce it? Imagine some orcs coating the floor of their entryway with something really sticky -- so sticky that the players have trouble moving. Instant loss of Dexterity bonuses. Or, coat the same floor with something slick. When the players keep falling down, they can't fight effectively. The monsters in question can unload a hail of arrows into the party while they fall down like the Three Stooges!
Take advantage of lair construction. Consider murder-holes and arrow-slits. If you have two or three sharpshooters hiding behind a stone wall with some arrow slits, they can wreak havoc on your party before they even have someone to respond to.
Make use of simple traps and ambushes so your monsters can get in flanking attacks, or separate party members so they can't assist each other.
Play dirty. Stun one of the weaker members of the party and take him/her hostage. Demand the PCs throw down their weapons or the player dies. If they do it (this rarely works, but it can still be fun to try!) then attack them ruthlessly while they're unarmed! They'll really end up hating these critters, even if they're a few levels below the party!
Make use of standard tactics. Don't stand in one room advancing rank upon rank to its death. Instead, have a fair portion of the monsters retreat to a better-defended spot. Have them pick up spears or javelins along the way and hurl them at the party as they advance. Retreat to a room beyond a secret door that your monsters know about. When the party follows you into the new room, surround them and cut off their escape.
If you use a few of these tactics, especially if you combine a couple, you could make a lair of kobolds difficult for a 10th level party.
In short, assume that the leader of the monsters has some sort of special knowledge of fighting or perhaps is more intelligent. Maybe he was enslaved by a human army commander, and learned a fair amount before he was able to escape. Make your monsters sneaky and take advantage of the terrain. Use guerilla tactics. Hurl a few Dispel Magic spells at the PCs, or use some magic yourself.
Be devious!
