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First, note that this process is never predictable. In most cases you can make a new piece in less time than you can spend re-glazing it, with much more predictable results. But sometimes there is that piece you can't part with and really want to re-glaze. Here are some things you can try to increase your success rate. The goal, of course, is to get the new glaze to stick to the old glaze.
Spray the piece with spray starch, let dry, then reglaze.
Spray the piece with sticky hairspray (usually the cheapest you can find), dry, reglaze.
Heat the piece first, with a heat gun or in the oven or kiln.
Brush white (Elmer's) glue on, let dry, reglaze.
Microwave the piece for 30 seconds. (Some potters say this makes a huge difference, and the piece doesn't need to actually get or stay hot)
Add some suspension agent to the glaze (CMC gum or Bentonite.)
Add some detergent / shampoo to the glaze (baby shampoo is good because it doesn't foam)
To improve your odds further, wash the pot first with ammonia or detergent, wearing rubber gloves, and don't touch it. The oils from your fingers can prevent glaze from sticking.
And... Don't use too much of anything. If you get the coating too thick, youmay prevent adhesion instead of encouraging it.