Whiskey has been steadily improving her lope, both speed and quality, by doing lots of hindquarters in, loping off, drive her under herself with lots of outside leg, then sit quietly when she gets it (even if its only a few strides) and give her her head. If she speeds up we lift the reins, lifting her shoulders up while spur pressure on the outside... then advance to asking her to slow with the spur pressure followed by the reins, then to just spur pressure.
We do lots of walk, trot, and stop back transitions and keep the lope work down as it is very hard on their bodies, and at first they simply don't have the muscle memory to hold a frame.
When Whiskey started to go too slow and started to lose quality of movement (troping) - we would speed up and gallop a few circles, then ask her to come back down to a collected lope... it is important to make sure they move clean over move slow.
It has taken about 5 months of consistent riding (4 to 5 times a week on average) to get her where she is starting to hold her frame for a class - it takes a lot of time to get them strong, so don't push her too fast or she'll sour at the lope and you will have bigger problems...
good luck and have fun!
There is no passion to be found playing small - in settling for a life that is less than the one you are capable of living. - Nelson Mandela