How do I do threshold training with heart rate only?

Me, clearly not working hard enough on a climb since I was able to snap this picture

  
Should  I shoot for 80% or 85% of my max heart rate?  And where should I start to get my max heart rate?


To answer your question about threshold heart rate, I think shooting for 85% is the best way to move forward.  To do this, you need to have a good conception of your max though, which can be difficult.  You could do 220 minus your age but this is really a vague approximation.  What I've done is go here:

http://www.brianmac.co.uk/maxhr.htm

and use the calculator at the bottom, and it'll give you a max HR based on a bunch of different methods.  Start with the highest result and calculate 85% of that.

Why start at the top end?

That's the most important factor in all of this....the reason you want to start at the top (even with the possibility that you may over-shoot) is because there is very little penalty for going too hard whereas going too easy puts you closer in smack dab in the middle of the "gray zone" which is a waste of your time.  If you overshoot and can't hold the effort for the prescribed threshold interval (let's say 15 minutes) then you've still worked hard at an above-threshold intensity and the trickle-down theory of training applies.  Meaning you still improved your threshold efficiency, except perhaps in not exactly the way you wanted to (you didn't get the duration in you were hoping for).

But you didn't under-shoot which will not boost your threshold or above-threshold power as the over-shoot would.

If you over-shoot, then the next interval (or next threshold workout) just bring the effort level down a beat or two and go from there.