Think of it this way - everything in horse training comes down to a horse responding to pressure. When you are working with her, apply pressure "as light as possible, as firm as necessary". Always start off with the least amount possible and build. Once you see the response you are asking for, immediately release pressure. This release is the horse's reward and how they will learn what you are asking.
For example, if you are teaching a horse to walk off under saddle, you would first squeeze lightly, then a little harder, then cluck, then use a crop to lightly encourage them to move. The instant the horse moves off, you would stop applying the aids. While you may have had to go through them up to the crop at the beginning, you will find that soon your horse will move off from just a light squeeze.
The key to working with a green or rescue horse, as with any horse, is patience and "horse sense" to understand how to set your horse up for success in training. This applies anywhere with horses, and the better you apply these principles, the more "trust" this mare will have towards you. This is true from basic halter training to teaching the piaffe or how to bow!
If you would like some advice on specific exercises, you can always contact me and I would be happy to help. I'm not entirely sure where your mare is in her training or what her personality is like from your post, so I can't really tell you exactly where you should start!