I previously wrote this for another site. Quick cut n paste:
You'll need a boot brush, a yellow cloth, a blowdrier and some wax. Beeswax is good but I like Nikwax. It has to be solid wax from a flat tin; no spray cans! If your boots are patent or of a light shade then the wax MUST BE SILICON-FREE.
-Clean your boots thoroughly.
-Using the yellow cloth, rub wax all over the bottom half of the upper in small circles until it fades in.
-Still with the yellow cloth, rub a thick splodge of wax into all the stitching lines so that there's a thick line of wax on the surface. Go particularly heavy on the area around the yellow stitching.
-Blowdry the boot until the wax melts. Be sure the blowdrier is on a medium setting. Don't turn it up all the way or you'll harm the leather.
-Leave overnight.
-In the morning, the wax will have settled around the joins. Blowdry it again.
-When wax dries, buff off the excess with light use of boot brush.
-Blowdry to seal.
Done. It isn't perfect, but it'll keep the weather out nicely.