Others have so much more experience doing the multi-boxing thing, but I've tried doing it a lot of different ways ever since the 1999 EQ days... so here's my outlook on things.

The main thing to do is map out your strategy for pulls and killing. If you are PL'ing your new account, you have to make sure that you damage the mob you are attacking to grey it out for your level 70 character. There are also some classes that are just more efficient at PL'ing other classes, most noticeably the healing classes (at least when doing mob grinding for XP).

So, for instance, when fighting melee mobs, you have to select your target, damage it in some way, then keep on auto-attack (or whatever) then using a macro start an attack with your level 70. For instance, a macro that does:

/assist Noob
/cast DamagingSpellOfChoice

tied to a hotkey on your main keyboard (preferrably a single keystroke, like number 0 or = on the main hotbar).

Obviously, things change if your level 70 is a melee or Hunter.

Plan out what to do in the case of multiple pulls. Your level 70 may have to take out one or two of the adds just so you don't die. Also, if you are grinding mobs for XP, I suggest sticking to yellow mobs as they will be the safest to pull and still survive if more than the expected number come. If you are doing mostly quests, you'll need to group up with your level 70 and hammer mobs for quest drops and kills, then go back to grinding with the noob character to level up skills. You can also do instance quests for good XP grouped with the level 70.

Hope that helps. And I probably got something wrong.

Pmortal (and W-Zmortal)
Bronzebeard