Most of my paintings are painted on thin plywood (primed). They get a bit warped if I do not frame them with some appropriate strips of timber. The larger the surface the larger the strips being used. My usual frame for a 48" x 36" has one diagonal strip as well for stiffness. In this case, I use 5/8" x 1" strips - or thereabouts.

Paintings I wish to keep for myself are framed by me. Other paintings are left unframed but painted white along the edges.
My own framing usually entails purchasing various decorated (profiled) strips of wood from the hardware store. I arrange them, stain, gold-leaf, and varnish/polish it, according to the composition of the painting. Here is an example:
