Different methods work for different horses, but one of the best things to try is making her work when she tries to move off when mounting. She will come to associate moving when you're mounting with work, and standing still while you're mounting with comfort and rest.
First thing I would do is make sure she's desensitized to the saddle and stirrups. Flap them against her sides a bit because with some horses, as soon as there's any kind of 'movement' or action involving the stirrups (putting your foot in), they decide to walk off. Do not stop flapping the stirrups until she stands still and relaxes. As soon as she stands still, relaxes (cocks a hind, drops her head, licks and chews, etc. - if she doesn't do any of these, wait for a minimum of 15 seconds), then stop and reward with pets and "good girls". Do this a few times on both sides. Then try mounting her, but go slow, just put your foot in the stirrup lightly and put a bit of weight into it. If she stands still, give lots of praise. If she tries walking off, immediately start circling her at an active pace and make her go around 15 times or so. It's best if you can actually lunge her in a circle by having a halter under her bridle for this part, but if not, just take the reins from over her head and make her do this at a jaunty walk. Stop and try again. Everytime she moves off when you mount, she has to go around in a circle 10-15 times at a pace YOU dictate - make sure it's active. She will come to learn that she'll be made to go out and work if she keeps moving when you mount. This will take patience and consistency on your part. The other alternative to the walking in a circle is backing them up pretty aggressively for about 20 steps (not lazy, slow steps - really make her move backwards like NOW). Everyone has their preferences and so do horses - see what method works best.
As soon as she does make an effort to stand when you mount, give her LOADS of pets and vocal praise. Then get down and put her back in her stall - now the reason I say get off and don't ride is because you want her to understand that her standing like a good girl will get her rest, comfort, reward, etc. If you put her into a hard workout after that, she'll get confused, because no matter what she does she's going to have to work. Next time you do this exercise, do the exact same steps, but don't put her away right after she stands still for mounting. Get off and re-mount a few times and then put her away. Then after this you can start riding her after you've mounted. But remember, sometimes horses will have a relapse of behaviour even if you've taken the time to train them, especially if you don't repeat and reinforce. So make sure everytime you mount her she HAS to stand still for you. After you've mounted, sit on her for a good minute before you ask her to walk out. She is NOT allowed to move until you ask her. If you ask her to move out too quickly after you've mounted, she will begin to anticipate this and start walking off again while you're in the middle of mounting or you've just mounted, so be careful.