I would not recommend using the alpha roll. You may be strong enough to over power her now, but you may not always be. Using force is not a way to get what you want from a dog.
Definitely look into using the NILIF program.
Here are a few links for you:
http://members.aol.com/dplantier/nilif.htm

http://www.k9deb.com/nilif.htm

http://www.k9force.net/nilif.html

Some tips for teaching down are when you are trying to lure her place your hand on her rump when she is sitting so she can't get up and slowly lower the treat to right in between her feet. You may have to at first just reward her for putting her head to the ground and not a full out down at first.
Another thing you can do with luring is kneel on the ground with one leg stretched out in front of you. Have her on one side of your knee and the treat on the other and lure her under so that she has to lower herself down to the ground to get the treat.
Another way is to stand her up on a table where she is unsure of jumping off and lower the treat to the edge.
Teaching the down from a stand may also work for her. When she is standing lower the treat to between her legs. If she goes into a "bow" stance you can give her the treat and gently press on her rump.

One final tip is to teach down after she's had an especially hard play session. If she's tired she's more likely to want to lay down.

Good luck!!