I have a horse that takes forever to warm up! Well, about 15 minutes. But it's a struggle for me to keep my mind of track during all that "boring" stuff! I'm no pro, but here's what works for me:
Sometimes, we lunge before riding. Just to get some extra spunk out, so pent up energy doesn't annoy me for the rest of the schooling session. It also depends on her mood. I can tell when we're tacking up what kind of horse I'm going to have that day.
Firstly, we walk. I hate it when I get on a horse and the first thing they do is get jiggy and want to trot off with you. I will force my horse to walk a lap or two, backing every time she tries to speed up. Because she's a pain and I like for her to suffer.
Than we gait slowly down the long sides of the arena. Lengthen and shorten her stride. Transition from walk to gait to halt and back down. Afterwards, we do some circles, spirals, square turns, and more circles. Circles are love.
After this, we must top it off with some cantering. Even when I'm not planning on working the canter a lot, she has to canter if she's going to gait well. Not sure why, but it does a lot of good. After a few laps of cantering, we're done and warmed up.