I usually press my hair once a month or once every other month. My stylist conditions my hair well, does a light blow out, and then flat irons my hair with a little oil. He usually uses a product called G Naturals, which is excellent, but expensive. I think that the most important things when pressing are to deep condition your hair, blow it as straight as you can get it, and then use a light oil for shine and protection.
Also, I think i remember reading some other raves about Dudley's Creme Press although I've never used it. Try doing a search for more options.