First of all, I'm flattered. I know I will get roasted by the regulars for your having called me out, but what the heck.
Second, if you do dedicated ham stuff, make sure you do it in balance with other stuff; balance of forces about the knee is very important. Also, the ham is a 2-joint muscle--it actually inserts in the upper part of the lower leg, and therefore crosses the knee joint--and is thus important for knee joint stability.
Third, about stretching--I know some people who respond very well to ham stretches (me), and some who respond very poorly, seeming to come out of the stretching period tighter than they went in. Maybe they don't know how to stretch, maybe they just have different physiology, I don't know--all I know is that if you do stretch, make sure that it works, and feels good. If you feel like your ham is knotting during or after the stretching, FORGET IT. Do some light dynamic stretching instead, in that case--light jogging, leg swings, etc--nothing dedicated to the hams.
Then, the actual exercises. Since you said you are already doing ham curls in the gym, I would recommend that you do them on the machine where you are lying on your front, if you can--as opposed to the machine where you are sitting.
Before I would recommend anything in particular, I would ask how old you are, what level of running/training you are at, if you have a reliable workout partner, what particular events you want to do, if you have had any relevant surgeries, and if you experience any recurring injuries relating to your lower body, or your hams in particular, and what facilities you have access to.
In general, ham curls can be good, back extensions, straight-leg extensions on the right machine, etc.. One of the best exercises I have seen involves a machine that allows you to start with your upper leg parallel to the ground, with the pad under the lower part of your upper leg, which you then press downward and rearward--it is actually a hip extension. It simulates a running motion very well if adjusted properly, and it works not only the hams but also the glutes and the adductors, at the same time, in a pretty balanced way.
As far as ham curls go, I personally find that they can be great for preventing injury, especially if you can exert most of the force in the range of motion from when the leg is fully extended to halfway flexed, then ease off on the force applied the rest of the way...don't try to get a dense "ham pump" from the exercise--in fact, don't "try" to pump through the whole range of motion at all.
More later, I have to go, but something to chew on in the meantime. I'm sure others will chime in and tell me how wrong I am in the interim!