What strategies do you use to get these things?
People are forgetful creatures. They leave things behind.
Or you can just take it straight from their home.
But make sure you follow the law 
There’s also the option of taking dirt from the grounds of their home.
Could get them to sign something then get there writing from them and use that for the process its easier then stealing clothing and its something that comes from there soul personally you take a bit of there soul already with there writing. There’s a reason why we’re told that Contracts are needed for Entities to do what they need to when it comes to doing certain things, because a persons writing holds more then there items ever could. Or even better there blood but if you can’t get that then there writing i would say is there best hope. Tell them you need there help writing something it always helps.
Underwear are the best.
If in school, I can always ask for a pencil and never return it.
Possessing as in you possessing the object?
Or more so poltergeist/other spirit possessing?
Like getting a hold of an object.
Ooohhh, sorry I got confused by the thread title.
Well I mean if your relatively close to said person, next time you hang out you could come up with something to ask them to borrow.
Or you could just “borrow” something… Although I don’t totally condone this one. Posses it, then return it. Anything from say a napkin, to a picture, to something else people don’t really use but will still have around…
I took couple of times trash of her front door in the night before the garbage man could took her trash…
Believe me you find very personal things 
I see what you’re asking and this is something I’ve been wondering myself because Possessing a living thing should be possible by switching your soul with the soul of that thing but to do it to a item which might not hold a personal soul of its own but fragments of another persons energy. So when say possessing the object besides the Energy left behind from the person you took it from which is Limited and has a time span what is there to posse your body. You got to think about it on a equivalent level so when possessing something that belongs to them you need to posses a item with a existing soul so it can latch onto your body well you posse it otherwise i think you might risk loosing your original body and not being able to go back because there’s nothing there to keep it going.