Lend a fellow DM a hand?

First of all, if your friends don't like planescape, something's wrong with them, so you should get new friends

Since i'm guessing that's not an option, here are a few other things you might try:

-After introducing them to the factions, make it possible for the PCs to join a faction...and then give the party quests originating from that faction, which will upon completion not only give material rewards, but will also increase that character's standing in the faction

-I would reccomend keeping the standard d&d races. They're a bit different from the "normal" ones, but they're still there

-Get some famous people to put in an appearance. Players always like to see well-known NPCs

-If they get themselves into a bad situation, present an unexpected portal as a way out.

-Throw in some Chaosmen...they always liven things up....

-Get a bullwhip...just in case my ideas don't work

My advice is not contrary to these guys, but additional.

Treat your party as you would a prime one. Give them a core point, or home base, or favorite inn, etc. Try to make this on Sigil or the Outlands. Then as the party gets used to the place doing piecemeal stuff, as touts, bodyguards, knights of the crosstrade, etc, eventually start moving them to Outlands adventures, remember, everything's out there. I recently started mine with an ankh of ressurection stolen from a minor shrine of the Egyptian pantheon in a small town, not far from the gate town to Mt. Celestia. The whole party was neutral or evil. It was rather good.

Once they get the function of the Outlands and Sigil down, give them a job they've already done, but send them to an Outer Plane to do it. Have an aasimar hire bodyguards for her trip from Mount Celestia to Ysgard by way of the Outlands. Then have their return trip go some entirely different way, like snip it off with a fast portal, or slow it down with a ride through the Oceanus. Let them get the feel of planes that match their own mindset.

Then pull the rug out from under them the moment they get cocky and overblown. Oh, you know all about the Outlands, huh? What about that Concordant Dragon with the army of rilmani moving towards Acheron? You know what that's all about, huh? Yeah, that's what I thought. Send them out of their comfort zone, into dangerous climates and lands, and push that survivability to the limit.

To temper this, the best cure for gung-ho-ness is comfort zones. Give that sorceress a day of lounging and shopping for new wands with no one picking a fight. That barbarian from Ysgard? Taphouses and a good sparring ring. Let that rogue spend some of those gains for once. And let the cleric feel a little love from his god or goddess. Make sure to make them feel like they really are off the job for a session or two. That makes the new, refreshed rush all the more exciting.

Pacing makes any game hectic or smooth. The rest is flavor text.

The only other thing I could say that hasn't been said is to NOT dumb down the feel of the planes. Don't be like, "Ok, Arborea is like, bigger than the prime and stuff." Really add details that make the players remember the atmosphere. In the Waste? Let them know that the plane makes them feel like they're underwater. In Elysium? Tell them it's quiet- REALLY PEACEFULLY quiet- like an afternoon during a snowfall. Notice how things seem quieter when that happens? No car noises or anything. Anyway, find an expressive analogy for every plane and make it stick so the PCs remember just being in a particular plane, not just what they did.