I've have had some great success lately with my newly created Mixed group (see sig). Granted, they're still in the Outlands, but they're getting out of it pretty quickly. Instance leveling is great, and I'm likely to just do the kill quests in every zone after I hit 70 so that I can hit Northrend with a little boost. But anyhow...

I'm going to have to echo what people are telling you so far in this thread. The separate controls and keybindings are much easier for me since I'm a full hardware multiboxer, but you can just keep the keybindings on each character type different enough so that the healer isn't doing something odd when you're doing something with the DPS or the tank. Also, any of the characters that can heal have macros that just target each party member, based on the order they appear on my screen (so that I don't have to worry about which button to hit - I just hit the third button if I want to target the character which is third down on my party list on my tank's screen).

Other than that, the basic things you're going to need simply boil down to the ability to move characters in combat (this includes strafing and turning). Also note that you may want to turn off any addon features that cause party members to follow you after they go out of combat. Generally, that can be troublesome in instances when a character, for various reasons, goes out of combat in the middle of a fight. Maybe this is less of a problem for others, but it's certainly something I dealt with.

Aside from that, plan professions well and all that stuff.