Best advice on the planet, bar none: Don't think about her head. The head and what it's doing are meaningless. It's all in the hindquarters.
Work first on getting her relaxed and responsive - accept whatever posture she gives you in that state. Get her forward and in front of your leg - a gentle squeeze means go, and until you tell her otherwise she needs to maintain that pace without you nagging her. This is the very basic layer of "d" dressage. Without forward, you have nothing to work with.
Look up the training scale if you haven't already - The foundation is relaxation and rhythm, and the top is collection. You can't have collection without all of the foundation components in place.
Over time, if your mare is moving correctly from her hindquarters forward, she will stretch forward over her topline. Long and Low exercises will be helpful.
This forum is an excellent resource, and there are some very experienced riders and trainers who can answer with more specifics than I can (I'm still a beginner myself putting the theory to practice).
I just recieved my copy of Lessons with Lendon
in the mail - tons of useful info in there. Also read Centered Riding
: Almost every "problem" a horse has with his posture is directly traceable to either painful tack or the rider. Fixing and improving position and effective aids is always beneficial. Jane Savoie has several video clips up on YouTube that have also been a great help to me. Chris Irwin's videos are up on State Line Tack's website, too, and are worth a watch.