I certainly feel like I'm not concentrating enough on various of my alts at times. I have a 65 bard (68 aa's), 65 druid (8 aa's), 64 cleric (about 25 aa's), and 61 necro (7 aa's).
Generally for exp I just swarmkite hobs & let 1 or 2 of the other chars suck up exp, but that gets boring real quick.
Once the necro hits 62, I have another viable option, 3-box aggro kiting in HoH, which should work great as long as autofollow doesn't screw up as much as it does in Valor, PoK and PoT.
Aside from that, I like doing LDoN's while 2-boxing the bard & necro (again, becomes more viable as necro advances), and will probably begin raiding more in the relatively near future, hopefully bringing the bard & necro. The real trick with that is everyone always wants me to bring my cleric, and they are BORING.
I too have had many several month breaks in EQ, I think in large part because I solo too much. The guilds I'm in are not raiding guilds (though one is becoming more so), so there isn't a lot to do other than LDoN groups, which get old real quick.
I guess the real trick is to try to not let yourself get into a rut. If you do one thing too much, you WILL get bored of the game. Try to branch out. Do new things. Help others get stuff even if you don't need anything from where you're going. If nothing else the change of scenery and activity can reinvigorate your interest in the game.
Anyway, that's just my 2cp.