There are a bunch of approaches here, but I can never remember any of them other than the one I use. I'm sure someone will chime in.
The wobble is usually from the case having a warp or from the battery not seating quite right (it was a poor design choice to put a foot on the battery).
What I do is determine which foot is higher than the others, then I position the PB on the edge of a table and GENTLY press down on the case on that corner. When I say gently, I mean it; imagine the amount of force needed to open a can of soda and translate that to a push instead of a pull. It really seems like you aren't doing anything.
That does the trick for me but the case will eventually warp again, whereupon you have to do the push trick again. Usually you have at least a month or so until that happens though.