KB has some of his own tips of course, but here's one that's never failed me:
PJR's (control the negative and don't bounce out of the stretch, do not let the upper arms come all the way to perpendicular to the floor either... You can lock out but you don't have to).
Those are the most elbow friendly extensions you get imo. Some DB's make them impossible to do though.
Another: Lying Tricep Extensions with a pullover motion (i.e. not skullcrushers) where you bring the bar down behind the head and basically do an EZ bar PJR (can go heavier here), though they are usually a teeny bit harder on the joints. May occasionally have to phase them out for a while.
Both have a good bit of long-head focus.
Doing those pullover+extensions (how far you go into the stretch is up to you, if you want to go real heavy, I would not go all the way down) with the bench in front of a cable stack with the right attachment (similar to EZ bar for me) can also work well.
-Use elbow sleeves from EFS or TK's...
-Warm/ramp up properly, do stretch exercises and exercises that are hard on the elbows after your other tricep work...
Things I can't do... Traditional skullcrushers/nosebreakers impinge something in my elbows... Straight bar extensions on the bench, odd on the tendon and impinge something... Lying DB Extensions unless I make it a pullover-extension and tbh they suck even then past the 70's-80's no matter what I do...
Overhead Extensions I can occasionally do but only with the long, heavy EZ bar (not the regular curl bar, the bigger one is like 50 lbs and both grip areas are spaced wider apart and angled more)
As far as tricep anatomy... The lateral and medial head do the same thing: Extend the elbow. They come from the humerus.
I haven't had much luck isolating either... They both come from the same tendon and insert into the same bone over only one joint.
That being said, elbows flared pressing seems to put more strain on the lateral head (just feeling it, can't say if it really makes a difference), especially BTN presses.
Medial head... Ok, technically you can hit it with pushdowns and all that same as the lateral head, but I found mine growing out only with heavier pressing exercises, esp. elbows in (might be because I'm much stronger with tucked elbows on the bench)... Pushdowns and such have done jackshit for it.
I'm not certain, but it seems that in some guys, the medial and long head are fused? At least that's what it looks like. I wonder if BBB can chime in and confirm whether that's possible and if that fused head then performs the functions of both in equal measure.
As it is, my long head get nothing out of any free-weight press. It just doesn't have to do much there and pure elbow extension/pressing only trains it (from one side and it seems that it's just not effective). I once thought that dips put a lot of strain on it, but it's just the long head being pushed against the lats... No dice.
Gotta say that pushdowns have never really done anything but give me a pump. I use them for warming up at most (usually not strict either, but as a pulldown+pushdown if that makes sense, just focusing on the long head instead of the lats for the pulldown part)
Long head spans all the way from the shoulder to the forearm, does not come from the humerus.
Mostly stabilizes the arm during strict pushdowns/extensions and so on, does basically the same thing as the lats to the arm. Thus a pullover motion combined with an extension part to stress it fully.
Smith wide reverse grip benching and IH presses (press towards feet as well as up) may be the only pressing variants (and pressing AGAINST the rack towards your feet on free-weight stuff)
that do anything for it because it's main function is being trained to a degree there, but I would not want to rely on those alone.