One way that taught me a lot. Go to a third world country and just for fun, negotiate retail items (baseball cap, tshirt, whatever). Shoot for 80% off. Can you do it? Then go for 90% off...

You'll quickly learn that you have two weapons: money in your pocket that the other guy wants, and the ability to walk away if you don't like the deal.

Everything else is fluff.

PS: another way to practice is go through timeshare presentations. Keep saying no. You'll be amazed how low the prices can get. You'll be handled by 2,3 or 4 different sales people, each with supposedly more authority and better deals to offer you. Keep saying no. You'll see :-)