Keeping local modifications

Hey, I'm keeping some local modifications to some portfiles to keep them working with a bunch of local modifications to the system. In order to do this, I copy the ports I modify to a local dports dir and make the changes there so they won't be lost when I do a selfupdate. Is there some way that I can make my locally modified ports totally replace the official ones? Just adding a Revision tag makes them impossible to distinguish in the list and I also see both versions. Is anyone else doing this and how do you usually do it? -- kent