Since there are so many, when I find one I like I write the specifics down on the pattern and if I have an extra piece of that plastic that you get with it I include that as well. By doing this I don't have to wonder what worked.
I always keep a piece of the plastic pinned to the interfacing in my stash so I know what it is.
If I am shopping for more I always take along that piece of plastic with me because when I get to the fabric store the interfacing all turns into a sea of white and I can't remember exactly which kind and what weight I liked before.
Many times the people at the quilt shop or local fabric store are able to help as well. I don't have much luck with advice from the big chain stores however as I don't think many of the people who work there are actually sew themselves.