Cannot update

I rooted my G1 and now I can't get it to update using JF's updates or even T-Mobile's OTA update. I have tried doing it manually using the SD card and that did not work. The error I get when using the SD card is something like "cannot find signature file" or something like that. I have named the update file just "update" and also tried "update.zip". The only reason I even put root on it is so I could tether it. Can someone please help. I would like to update it but still keep root if possible.