Something that always works for me is to knit the first 2 (or 3) stitches with the working yarn and the tail end of the cast on row held together. It makes a nice tight join... after 2 or 3 stitches, justdrop the tail and keep working with the working yarn... when you get back to the beginning of the round, don't forgetthat the first 2 or 3 stitches were done with a double thread, so you don't accidently add some stitches.
Yeah, I learned how to do that circular join on my socks and have always used it since.
It's called TRADING PLACES in the Stitch n'Bitch book. You're putting the first stitch on the left needle over on the right needle and then the first one on the right the original first stich which is now one in, over on the left. as stated already! (sorry to be redundant, but the name helps I think...Trading Places...lets you visualize it completely)
It always works for me and it's nice and tight and smooth.