My favorite method is the working trot. Encourage her to stretch long and low and use your legs on her body to encourage a little lift. Lots of circles and figure eights. Trotting over ground poles and cavaletti are also excellent for strengthening back and abdominal muscles.
To get a proper working trot, the hind end has to engage and you can feel the difference. It might take quite a bit of leg at first, if she's not used to it.
If the problem is relaxation, as in, she isn't relaxed at the lope, you can teach her to do it. Trot a big circle and spiral in, letting her slow down as the circle gets smaller. When she finally stops, let her rest for a couple minutes. When she gets the hang of it, try it at the lope. It might take a while, but she will come to understand.
Once she does, you can use it by starting a spiral every time she speeds up. When she relaxes and slows down, take her back out. Make sure to give her a break when she gives you what you want. Best reward in the world.
Also, pay attention to how you're asking for the half-halt at the lope. You may be doing something different than other gaits without realizing it. And practice transitions between gaits, as well as within them.
That was kind of convoluted, sorry. Hope this helps, though.
Learning never stops