The biggest problem I have found in practicing moving reloads is that I am constantly having to clean my mags. If your range is like mine, sandy at best and mucky at worst, the process can get old real fast. Even, if you have the luxury of a concrete floor, your mag feed lips can get banged up pretty quick.
Try the following warm-up drill to reinforce prepping the trigger and practicing your reloads. You will also find that you will get a good work out!
Position two full targets 10 yards ahead and about 5-7 yards apart. The "X" represents the start/end positions, and the "T" represents the targets:
Start by loading your pistol as you normally would at the "load and make ready command". Now comes the hard part, remove your mag from your pistol and insert it into your first mag pouch. Now you are ready for the warm-up drill, i.e. chamber loaded only, safety on and pistol holstered! I know that this sounds strange but try it.
Facing down range at position X1, draw your pistol and prep the trigger (i.e. take up the slack only) while aiming at target T1. Now from position X1, run to position X2 while performing a reload. As always while running or reloading, keep your finger OUT of the trigger and watch your 90 degree break. When coming into position X2, fire 2 shots into the second target T2 (A's) and stop. [Note: if while prepping, you have over prepped and fired a round, just remember to index your finger along the slide and rack the pistol after your reload.].
Remove your mag and reinsert it back into the first mag position. Now you are ready to do the drill again, from position X2 to X1. Keep repeating this drill, going back and forth. Within a short period of time, you will have practiced moving reloads without having to clean one mag! After about 10 minutes of this, try moving one of the positions forward to practice reloads while moving forward. Another variation of this drill would be to place a port or doorway in the second position. If at all possible, try to replicate what you see in a match, i.e. running left to right, right to left, low ports, shooting around a barricade, etc. Continue this drill by reloading from your second mag pouch, from your third mag pouch, etc.
Now that you are warmed up, try doing the reloads the regular way. This time fire 2 shots into the first target from the first position, perform a reload and fire 2 shots into your second target in the second position. Ideally this drill is performed with the assistance of your shooting buddy, with timer in hand! Check the timer for the split between your 2nd and 3rd shots - this will be your reload time. Do the same drill without the mag change, and compare times. Your ultimate goal is to be as fast with the mag change as without!!