Is this a new development? Did she used to move out under saddle well and recently start freezing up? If so, I would be first ferreting out what changed (tack, feed, turnout, anything that is now different).
Assuming that the behavior can't be linked to a physical issue, I'd say this is just a hole in the mare's training: something that either was never properly understood, or perhaps inadvertently taught through ineffective communication.
Always use the progression of seat, then leg, and then stick. Do not kick or spur for this, as that will only cause her to shorten her stride, not move out. Think about moving forward - that will automatically put your seat into use, but remember not to lean forward. If she doesn't move forward off of the thought, apply your legs in a firm squeeze. If she doesn't go off of the squeeze, maintain that leg pressure and give her a firm tap with something, whether it's a mecate, dressage whip, NH Stick du jour, or a sapling. I too would advise against irritating the horse into moving - ask, suggest, and tell. Don't nag about it, just get your point across, and then go right back to neutral once she responds. A well timed and consistent release will get the point across quickly and without stress. The most important thing to remember is to be effective in the saddle - I've witnessed the "irritation technique" make some darn near bombproof horses.
I have personally used the method of smacking my own leg to motivate a horse to go forward before, but I honestly can't recommend it. For all the more it helped, I had a wicked looking bruise on my thigh. Perhaps I was doing it wrong.
If you go that route, I'd advise wearing a sturdy pair of chaps for the exercise.