Reg-
There are several articles that have been published in Bass Player magazine recently on fretless playing, written by Ed Friedland, that are very good, and give some great suggestions. Surf on over to
http://bassplayer.com and see if you can track them down.
Failing that, what I did to get my intonation together at first was to play major scales on one string against an open string, very slowly. For example, I'd play the notes in a D major scale on the G string, while playing the open D string at the same time. After a while you just start getting a "feel" for where the in-tune notes on your fingerboard are, it's a combination of muscle memory and trusting your ear. Hope that's helpful.