Adding a new computer with a rear wheel sensor to my bike has been a nice addition. Since it has a cadence feature it's easier to keep my RPMs on track while I'm riding on the road and in the trainer. In addition, I've got better points of reference for my trainer rides.
For example, when I rode this morning I broke the body of the ride into 10 minute segments, with 6 minutes harder, 4 minutes easier. To esthablish a consistent baseline for the hard segments, I simply kept the indicated speed above 16 MPH. Purely arbitrary, but it seemed to work pretty well. When I was maintaining that speed, my HR moved up into the 70-75% range and stayed there pretty consistently. That puts my HR at about 140-145 for relatively long stretches -- a good effort level, but not one that'll shatter me -- perfect for building a good endurance base.
I'll experiment as time goes on with either increasing the amount of time in that zone, or increasing the minimum speed level on the hard efforts, depending on the training benefit I'm looking for.
I've got a 1/2 hour run on the schedule for this afternoon/evening. The weather's pretty nice here today (sunny, high 40s, medium wind), so it should be a pleasant time.
My training log at Beginner Triathlete.