I like the transitions advice, but I'm going to describe this lesson I had not too long ago, to round out the advice.
I'd asked for help on exactly the same thing, how to get my horse to round. I picked up a trot, and was told to shorten my reins to get a nice contact. My horse, especially at that time, would lift her head. Then I was told to trot "more." So we went faster. "More." So we went even more. "DON'T LENGTHEN THE REINS!" My horse started to pull heavily on them. "MORE!" We went faster. "Don't worry, just get a bigger trot." I gasp, "There yet?" My instructor says: "Almost."
Then suddenly it happened, she came through the back, her whole body seemed to soften, and she was just a nice, soft contact in the reins! It was incredible; still flying around about as fast as we could.
Later (the lesson only lasted about 20 exhausting minutes) I told the instructor that it was awfully hard, to trot around at, on a scale of 1 to 10, a 10! He said she would get stronger, and then it wouldn't be her 10.
This is not a training method I can endure, and neither is the footing good enough where I ride at home. However, when I have problems, it's usually because my horse is not forward enough, so I'll start this "MORE" exercise. I must admit, it has never failed to improve matters.