Re: Wyoming trip in two weeks!
Davo said what I would say. The local shop should be able to guide you with the specifics. I always bring a decent selection (couple sizes, couple color combinations) of the basic wets and dries and it tends to work pretty consistently on western freestone streams. Coldwater streams generally have at least a couple species of either mayflies, caddis flies, stone flies, and things like scuds, leeches, sculpins, grasshoppers, etc. So if you cycle through the basics, you can usually get lucky and find a pattern they're willing to hit on. In general, I've found the pheasant tail nymph, elk hair caddis, and woolly bugger patterns to be enormously successful on western freestone streams. If the stream you're on has relatively little fishing pressure, fly selection probably won't be as much of a problem as you might think out there. High altitude trout tend to hit on pretty much anything remotely resembling a protein source if it's presented naturally enough.
The other flies, n., pl.
1. dry flies, nymphs, emergers, terrestrials, streamers, etc.
2. What I use when a black #10 woolly bugger isn't catching.