So my advice is to find a buddy, start early, and work together often. A great team of two can tackle almost anything in Azeroth, but make sure to organize yourself-- both of you will need to level up certain skills (first aid!), but there's a few professions that complement each other very well-- blacksmithing and engineering or jewelcrafting, for instance. Group quests can sometimes be tough with two, but if your classes complement each other and you're both experienced, it makes things easier for everyone.
Now, if you already have someone and need ideas for classes, we've got you covered. For actually finding someone in the first place, the LFG system has grown on me a little bit-- if you're in a zone and looking for help, it's easy to throw up your LFG flag, and it usually grabs a few whispers. Guildies are always good-- I know a few guilds (including mine) are going to be leveling up together, at least to 70. It's too bad, though, that there isn't a service somewhere for just hooking up people for leveling. Oh, what the heck, let's start one here-- if you're starting a new toon (or continuing an old one) for the Burning Crusade and you need a leveling buddy, put your name, server, and faction in the comments here, or just drop a whisper to the people who have already posted. To Outland!