While I think Cheries post makes brilliant sense, I have tried a different method that seems to work. First, I do lots of trotting first, before I ever ask for the lope. Serpentines, circles, trot for a few miles down the road, whatever works. Wear the edge off, you don't want to approach a problem with a fresh, frisky horse.
Next, as was mentioned by another poster, try asking for a lope up hill. I find if you can set a horse up for success the first few times it will start a good pattern of behaviour.
If it is a truly learned behaviour, with the horse determined to buck the rider off, I find shutting the horse down and then making them work HARD gets the point across. I can't ride a bucking horse. Its a fact I've come to accept. Spook violently? No problem. Unexpectedly leap over things? Easy. Spin, bolt, crowhop, I can ride them out, no problem, but if a horse goes to bucking, off I come, usually in a broken heap.
So first, on the ground. If she bucks on the lunge, this is your first problem. A horse should NEVER be allowed to buck while being worked. Not the first saddling, not just to 'get the sillies out', never. When she threatens to buck on the lunge/round pen, change directions repeatedly, work her hard for a minute, and then ask for a lope again. When she is loping properly on the lunge consistently, do the same under saddle. When she starts/threatens to buck, one rein stop, disengage the hind quarters, then put her to work, hard, lots of direction changes at a good hard trot, then ask for a lope again. Horses like to take the easy way out if they can, and once she learns that bucking = more work, she'll give up.