How old is she?
She is sour, annoyed with people, and knows she can get away with just "plodding around" because no one has ever asked her to do much more than that. Give her new obstacles to overcome, maybe do some games to get her thinking. Try to teach her something new, like to sidepass, or perfect something she isn't very good at. Play with her on the ground, get those feet moving. But don't push push push because her mindset is, "Oh yeah? Who's going to make me?" Don't make her, ask her! :] Make the right thing easy, and the wrong thing difficult. Ask her to do less than she offers. After a while she'll want to do more than you ask. :]
Use a lot of variety.
Working with your horse should be like dancing: Someone must lead, but you must both work together to accomplish what you want.