So this is a process i've been going through this year, what we're doing at the moment is that when it comes to sparring, toe holds, knee bars and footlocks are taken to tap, if the person can get the heel hook grip securely in place then they tell the other person and that stops the round. Even getting to that point, things will be pretty tight so it doesn't tend to come as a surprise to the other person. Essentially we do everything except the final elbow lift and hip engagement. It becomes like anything else, where you start to recognise the lines of defence as they've broken them, conceptually it's not much different to:
"aww shit, my elbow is out, crap my arm is isolated, damn he's got a kimura"
just that it's
"aww shit he's got DOA, crap i can't clear the knee line, damn he's got the grip"

We also start with one person butt scooting as that's the most likely entry a leg locker will use against a non-leg locker, so you can do yourself a huge favour and make your standing passes part of your A game and not have to rely on the escapes.

Mmmm yea, my best leg lock entries come from top, I'm more comfortable there. I've been drilling a lot of inversions and things of that nature. Definitely tough to get used to. I've also been studying the instructionals of Eddie Cummings and Robert Degle. They've got some nice stuff when it comes to defense and counter leg locking.

Heel hooks scare me, one of the techniques I can't see myself ever using in BJJ because I'd be much too scared of the other person hurting themselves. BUT the best instruction I've seen on them is easily Charles Harriott. I enjoyed his class on leglocks, so much so that I brought him to my gym for a seminar, to teach an extended version of the same thing. Which goes to show how well he taught it, given how utterly terrified of heel hooks I am.

He's got a DVD out on the topic, plus there are a few clips up. His attention to safety was especially impressive. I don't know of any other instructor who is able to 'defuse' the terror of leglocks as well as Charles. He showed a ton of drills for heel hooks too, so I'd assume those are also on the DVD he did, 'Leglocks 101' IIRC. Here's the clip of him talking about leglock safety :

https://www.youtube.com/w[...]wEcswXry9o

Learn your defenses. Roll with training partners who know leg lock defense. If your partner doesn’t know the defense, simply play catch and release. Their ego may damage their knee but that’s no reason for you to hold on to prove a point. A well versed leg locker should be able to work their control and positions on an opponent without injuring them. If you continue to catch someone in a heel hook, show them the proper heel slip, running man, or stacking techniques to defend properly.

Hi! Check out old school gradppling DVD series.

I have a couple of rolling partners that I trust and that also know heel hooks, knee bars, etc...
We'll roll to finish with no injuries, but we've put in the time together to really understand where the line is and how to be safe
Initially we rolled to the point that someone had control of a foot, stopped, and worked out what was going on
Over time we've progressed
I'll hit leg lock positions in rolling on those that are less well versed, but not look to finish in case they do something silly

+1 for Leglocks 101 from Charles Harriot

I teach them and have my students drill them but do not allow them to be applied. I just want them to get a working knowledge of their application and become comfortable setting them up. I only roll with heel hooks with a few trusted black belts that are versed in applying them and defending them. Not worth the liability in regular classes.
Alot of BJJ instructors avoid them altogether. That is a disservice to their students.