I've tracked my RHR for years and have been pretty happy with it, but I'm curious if anyone else does the same.
When I was in my late teens, early 20's, training for road racing, the lowest my resting heart rate got was 42, and my average was about 46. That's measuring pulse at the carotid for a full minute upon first sitting up in the morning. If I had a good relaxed heart rate I would know I could put in a hard day of training without worries; if my heart rate was 50 or 60 I would know my body was still recovering from past training, and to take it easy.
Of course all sorts of things can come into play with heart rates, and that's more of a guideline. Doing this over years gave me a good base to judge from. I am 44 now, and pretty pleased that I got down to an RHR of 48 over last summer. Its more like 52 now, taking it a little easy through the rough winter. There was an interval when I smoked moderately and drank and slacked off generally, and I had an RHR of 60+, not to mention some chest pain (though I blame George Bush for that ) before I adopted better habits again.
The lowest rate I have heard of is Miguel Indurain, who I read had an RHR of 28. Anyone else use their RHR as a guide for training? I got the technique from some book back in the 70's, and I think it has served me well.