Setting up a rain fly is relatively easy in perfect weather, the trick however is to set it up in the rain.  Hence the name rain fly.  Doing this in a downpour and wind can be a real pain in the a$$.  The last thing you want to do is chase down flapping guy lines and trying to tie tricky knots.

Tips

1. Buy a pair of Hennessy Hammock Snake Skins and add them to your rain fly set up. They will help keep your rain fly under control and somewhat dry in bad weather.
2. Buy a set of Nite Ize Figure 9 Carabiners  to lash guy lines quickly around a tree.

While I enjoy using my knot tying techniques as much as anyone, sometimes it just makes sense to incorporate a few methods to help out.