I only use spurs because my horse needs a strong leg and I am still working on my riding muscles, so I only use them for a temporary tool.
I trained my horse to respond to my body position. He catches on extremely quickly so it didn't take very long to do. There is a video on it somewhere on youtube.
I started teaching him at the trot when I sit forward and also shorten my reins and slide them up his neck to give him the "go" cue. Then I would sit back and loosen reins and rest them right in front of my horn and say walk.
When he started grasping that I used it to transition from the canter to the walk, and with his canter it doesn't take much to push him into a gallop. If your horse is different you might have to quirk it a little.
But every time I position my reins/body he knows exactly what to do. I can raise my hands,shorten my reins and he is ready to take off. I had to use my spur to give him the idea I wanted him to start taking off from the first barrel to the second,etc.
I usually just search alot of pro barrel racer instructional videos on youtube until I find one I can relate to the most. All of them are pretty great :)