I echo what Eric said about initiative cards. I've used the system for the last two years, and it really helps out a lot.
The simple fact of the matter is that combat takes time, especially with large numbers of combatants, their level notwithstanding. I try to find everything I can to make combat move swiftly. With 6 PC's and one NPC, the last combat between the party, a Hezrou cleric, a Vrock, an Imp, and 4 Gladiatrixes (from Legions of Hell) took a total of about 7 rounds and just over an hour in play time. To keep combats moving quickly, here are some things that I
would suggest:
- Use initiative cards, or another fast and easy system
- Have players look up spells/abilities prior to use
- Be familiar with monster abilities prior to combat
- Have a rough idea of monster tactics prior to combat
- Be willing to make on-the-fly rulings. Don't quibble over details if you're having a hard time coming up with the rule.
- Don't correct small mistakes retroactively, just remember for the next time they come up. This applies to both DM and player mistakes.
- Read the combat rules inside and out until you are familiar with them
- Keep combat descriptions brief unless describing a dramatic event. Do
NOT eliminate descriptions entirely, just don't go overboard for a simple swing of the sword.
To keep combats deadly, well, I continually need to scale back my encounters to avoid a TPK, so I'm not sure why you're having a problem with this. Just use a variety of challenging opponents, and use multiple opponents. A variety of challenges is important, so that the party can't get comfortable in one mode of operation. They should always be kept on their toes. Try mixing it up with one big encounter one day, many smaller encounters the next. One big opponent one battle, many lesser opponents the next. Use opponent spellcasters. Use heavy hitters. Use the full spectrum of creatures available to you, with appropriate encounter levels, and you shouldn't have a problem. In order to get the most out of your monsters, spend some time out of game thinking about the most devestating tactics available them, and then use them!
To keep combats interesting, here is what I've found works:
- Vary the battlefield. Interesting terrain makes even hum-drum encounters exciting. Not every battle needs to be fought on a cliff face over a chasm filled with flesh-seeking worms, but there's a big difference between an empty hallway and a hallway with arches every 20', torches in sconces that can be torn off the wall and used as weapons, as well as a section of rubble that fell from the collapsing roof.
- Try to include challenges for all of the PC's. Not every encounter, mind you, but in general. Include some wimpy opponents along with the big bad demon in order to harass the weaker PC's. Include hard-to-reach opponents for the archers and spell-casters to deal with. Include heavy melee hitters for the fighters to deal with.
- Try to make encounters relevant to the plot. Whether the plot is "clear and loot the dungeon" or "save the princess from her half-fiend uncle," relevant encounters are more exciting. "Survive this!" can sometimes be relevant to the story, too

- Describe the action. When a PC kills a foe, cleaves through another, and downs a third with his last attack, spend a few seconds to give such a remarkable feat some attention.
- Describe the opponents. Instead of "the orc barbarian moves here and attacks", try "the scarred orc moves closer, his muscles rippling. He howls with rage and tries to take your head off."
- Not every combat needs to push the party to the limits. It's ok for the PC's to flex their muscles occasionally, just as long as the combat is brief. Your 8th level party may get quite a bit of satisfaction from laying down the smack on three ogres. Your 18th level party will get very bored when they have to lay the smack down on three separate waves of 32 ogres each.
Hmm, that's it for now. It's worked for me, hope it works for you.