Another overly simplified vague question. Best training aid?
A gun to shoot (in conjunction with Ammo to shoot)
A place to shoot
Targets to be shot
A program to develop one's skills
Associated equipment (holsters, mags, etc.)
Safety gear (ear and eye protection)
Professional evaluation every so often - classes to learn new skills and hone old skills, especially those forgotten or that have gotten sloppy.
Aside from the normal marksmanship and speed drills people often do, every so often I like to dedicate a range visit to working on trigger pull, very controlled trigger recent allowing for no slack, follow-up shot. This helps in double tap or other follow-up shot routines, helps to train out some of the tendency to let the trigger out to far and then mashing/slapping the trigger on the follow-up that will often throw off accuracy.
Dry firing is also very important. While not as important, but very helpful, I have a non-firing 1911 with an unbeveled mag well and like to practice doing tactical reloads (w/snap caps in the mags) over and over, such as during a TV program where my vision is focussed on the screen and not looking at the gun. After reading various posts where people have shot TVs and focal points on the walls, having a disabled gun is a good aid for the drill. I do two different drills. One is simply swapping out the mags over and over and over again. This provides training for keeping one's eyes directed toward the threat or where you expect the threat to come from while recharging the gun. It is a really good workout for the changing hand and helps develop coordination for the skill as well. It is a good drill for the strong arm as it involves keeping the gun out for a prolonged period of time. With this, I try to keep the gun pointed more down range as opposed to almost sideways as some people will do. The second drill is working the tactical reload from the mag holster and going through the full sequence, either pocketing the 'partially spent' mag in a pocket or in the waistband.
I also like to load up all my magazines and put in occasional snap caps, then randomly select a mag to go in the gun and spares in the mag pouch and do my normal shooting and working to clear malfunctions with tap-racks that will crop up in seemingly random intervals. It is one thing to know you will have to do a tap-rack and another to be surprised in the middle of a drill when the hammer falls and nothing happens.
I am going to have to give some consideration to the dry firing and treadmill idea. I take it that they are used together? That sounds like it could be a really cool way to work on keeping a steady sight picture while moving. From there, added the tactical reload drill would also be really good.