[Michael Chandra]

Eh, you can ask politely and give them offerings. Treat them with respect. I know I did. Including very respectfully overchanneling a Force 8 Great Form Guardian Spirit, who pretty much was granted the wheel after that.

As for asking them to possess something: You could ask them 'please buff me with the following spell, I prepared this proper attire for you to Possess' as a Service. Or 'please provide me with gunfire and counterspelling support, I bribed hired a few hobos mercenaries to serve as hosts' as two Services.

I might have used my elegant walking cane as one of the vessels? I forget really, it's been a few years since I played that character.

Vodou is the best tradition by the way: It scores ALL the special useful spirit powers. Task, Guardian, Man, Water...

[Beta]

There is the factor of bargaining with spirits outside of the strict rules (depending on your GM).  "I know this is not what you like, but I will burn a great deal of offerings to you in thanks" (sacrifice a bunch of reagents) or "If you will occupy this (thing) for me, I will later let you take over my body for a night of music, drinking, and dancing" (preferably under the supervision of a trusted friend!)

[Michael Chandra]

Triple been there done that with that character! Plus whenever I was Channeling, it was liquor time!

(Incidentally, playing a blind mage with Detect Enemies Extended is fuuuuuun!)