AF3+2 is the main thing. Specifically, feet, body, and head. With those three pieces, plus a decent staff (perp -3 or more) and Evoker's Ring you should be getting MP back with any avatar out. If you have magian perpetuation staff, you'll probably get 2-3 MP/tick back. The only thing I use which can be annoying to get is the refresh Moonshade Earring which brings me up to 4 MP/tick with avatar out (7 if I cast Refresh). There are also some pants which give refresh, but they're even harder to obtain.
To go beyond that, the only real way to do it is with atma that give refresh. For example in Abyssea if a SMN has trouble keeping MP up they will often use Minikin atma for 10 MP/tick refresh.
____________________________Pergatory (Asura, Kupo LS) WHM99 ~ SMN99 ~ DRK99 "If you want to know me, look inside your heart." -Lao Tzu