I just ran the standard Resveratrol HPLC vials again. I noticed that it is already converting to CIS. I know that people are using resveratrol + Ethanol. Maybe to max the trans content make your mix on a weekly or biweekly basis.
I dissolve only as much as I'm going to use right then. It doesn't take that long to dissolve it, and compounds never last as long in solution as they do in solid phase. I have little worry any more about the stability of solid phase resveratrol as long as you aren't doing something stupid with it, like leaving it in the sun all day. Solutions are a whole different story though.