Deepthroating and/or gagging produces a great deal of saliva, which makes for great natural lube. Lead in with some messy fellatio and you're all set. This works either way, whether the one being fellatiated is the uke or seme. If the seme, then it's pretty straightforward. If the uke, let the gravity do the work and the saliva will get to the needed area eventually. Or, if you want to get kinky, you can always go for direct application.