Anyone else who tried to harness train their cat?

I think that if you are teaching a cat, or even a dog, to follow on a leash; it is important to start out ahead of time, and work them into it gradually, rather than just putting the harness on and taking them out for a walk.
If you put on just the harness, and leave the cat in a familiar place with it, and be sure that the cat can't hurt him/herself; then pretty soon the cat gets used to wearing the harness.

Next, attach the leash to the harness, and let the cat get used to dragging the leash around.
Once they are not afraid of this; you can pick up the leash and follow the cat around. Gently, start teaching them to come along when you change directions, calling them as you do this.
Only after the cat has become adept at walking on the leash should you take them outside of the house.
By then, you will have learned exacly where to adjust the harnesss so they can't wiggle out of it easily.
When you first go outside with your cat, stay in the yard, where the cat feels safe.