I think the most positive impact I've had lately has been my ability to help people quickly complete quests that would otherwise be very problematic for them or that would require multiple people to help with.

I'd suggest some themes along the lines of "Would you like to help your friends & guildies complete the most challenging quests in the game?" or "Want to be the Most Valuable Person in your guild and on your server - try multiboxing"

E.g. Currently in classic, rogues need to collect 25 war beads from elite ogres as part of the T0.5 quest chain. The drop rate seems to correspond with how difficult the ogre is to kill. (open world Ogres have a bad drop rate, but the ones in DM:N seem to be about 50%) So our 1st rogue started trying to get multiple groups for tribute runs, but that would have taken him 2-3 full dungeon runs - likely each with a different group of PUGs. Instead I helped and we just spend 15min clearing the entire entry way and he was done quick and easy.

I've also left groups of 3 level 20-something Locks all over the place for quick summons - really helps getting world buffs and stuff. (hint - be sure to leave in places where you can easily refill on shards from time to time. So don't leave them by the felwood flight point songflower, but instead leave them by the one close to ashenvale)

I've think I've relatively quickly gone from - "there's that weird guy with all the mages following him. He says he's "multiboxing", but we all know he's just botting" to "Hey, I know I know a guy that can help with that" to "Wow, that's awesome - how can I do that?" I have one other guy in our guild dual-boxing and another guy just got his 3rd account.

(Its almost kinda annoying how many people I now have asking for help all the time, but I guess its nice being on so many people's friend list)