Max HPs each level.
Lose altogether or half the damage from area of affect spells. Maybe bump them up a spell level, (i.e. Fireball would be 4th level).
Do fewer, but more meaningful combats.
If you want to slow the PCs down, send out the one hit wonders, but with the best equiptment and tactics. If the PCs are 10th level, several 5th level NPCs would be a distraction, going down in one or two hits. So instead bring out 20 1st level NPCs. They won't have any greater staying power, but with more of them, they will take longer to kill. They will also do little to harm the PCs, so they will have ample time to deal with them or run if the fight isn't going well.
For important encounters, (say the BBEG), force the PCs to pull out all the stops or lose. Many of the players I DM for simply don't use potions and scrolls, and save them for trading fodder. Don't let your players take such things for granted. One way of doing this might be to not award such things as a potion of aid as part of a treasure, but to save it. Then, when you knoe the PCs are about to enter a tough encounter, have the kindly old apothacary, a total non combatant, suggest/give/force the potion on them. Instead of giving them treasure, give them the chance to make friends of low level NPCs with useful abilities. This also gives you more people to potentially hold for ransom, use to learn secrets about your PCs, or stab them in the back themselves if things are gettng dull.