This is something we're working on right now too.
A really good schooling exercise that I've been doing is to go round the school on the inside track. Rather than bend to go round the corners, you do a turn about the forehand, so your horse keeps her front limbs stationery and she just moves her hinds across to turn a right angle. We walk the short side of the arena, but trot the long side, go into trot IMMEDIATELY after the turn about the forehand we trot the long side of the school.
It really works for us. Logic goes off into trot like there's a giant spring under her because she's still concentrating on using her hinds and then she maintains it for the full long side of the school (back into walk for a few strides to do the whole thing again).
We still need to work on consistently maintaining that impulsion though