Well all kinds of basic things can help. Lateral flexion and hip disengagements for softness. Then I like to do lateral flexion at the walk. Then I like to pressure and release in order to get the head down where I like it. Sensitizing the horse to the bit so it takes less and less pressure and the horse leaves the head a longer amount of time.
Circles are a good idea to do at the same time. If the gait is too fast, circle with softness in the bend to even out the head and speed. Once you get to a slower, softer jog, go back to the straight line. In a straight line you can also hold your horse collected and push him up into the bit to get him to round up and slow down. Remember to stay sensitive to the bit and encourage self carriage with big releases. I would also invest in learning to leg yield/two track at the walk and jog.
It's also wise to teach your horse half-halt cues through your knees. I usually do this first at the walk. If I want a slower walk I'll push with my knees. They won't know what to do at first, so you go right into the back-up. After a while they associate the knee cue with backing up, so they slow their pace. You can get a nice crawly walk or a subtle decrease in the jog speed this way.
And don't forget to work both ends of the spectrum. I do a long of fast trotting, both "long and low" and collected up. The rounded fast trotting will help to engage muscles and push through for softness. I also want to have an amazing back-up that I can do without reins. I usually cue by bouncing my legs, then pulling. Once they associate rhythmic leg movement, they usually start to step back on their own, and it's up to you to moderate speed.
It's all about softness, sensitivity, and speed control. If you look into Dressage a lot of it will help you out as well.