She pulls. You drop your reins. Let her fall on her head it'll teach her a good lesson. As Mercedes said, she can't pull against nothing ;)
If she goes to take off, spin her with one rein, take hold and yank it to you chest and run her in a circle until she calms down. Then ask again with no rein. Takes off, spin her. Give her a disincentive to run on you.
You could go the other way if you're brave enough and if she wants to take off, let her. Kick her in the guts and run her flat out till she's pooped, then ask her to come back to walk and if she doesn't come back, kick her and run her again. She'll realise that when she takes to option to walk, life is a lot more comfortable.
Just a different way of thinking about it rather than leaping straight at saying throw a martingale on her... as this will not really alleviate the problem, you will still be pulling against her which will still give her something to lean against, obviously.
Particularly if you've put her in draw reins and she's learnt to suck back to evade. That is a hell of a problem to resolve if she gets into that habit so I would definitely stay away from anything that is intended to 'get/keep her head down'.