Summarize:

Use a mirror, record yourself, make bid calls aloud and use your skills in public. If you went to school for auctioneering, they'll get you started on this. Bid calling is an art, as are describing auction items and dealing with customers. Improve your skills by practicing them regularly. Bid calling is obviously the most visible part of the auctioneer's job, but it's only a very small portion of it. While it does matter, you also have to be charismatic, a great marketer, and have good administration skills to boot. A good auctioneer is more than just someone who knows their product and can speak ridiculously fast and still maintain clarity. They're entertaining the entire room! An auctioneer that charms their audience can drive prices up 20%. Don't get the crowd buzzing and the auction is a bust.  If it wasn't clear, you'll be working on commission most of the time. So the more money you get your crowd to pay, the better off you'll be. So you gotta work it! To a certain extent, you'll be coaxing your audience to pay more. People will automatically start to get hesitant as the prices rise -- it's your job to be persuasive and convince them (without them really knowing it) that they're making a good decision by upping the ante. A good auctioneer is often quite classy. You may be working with items that cost a pretty penny and your demeanor should match. Bidders are not "the woman with the orange hat over there" and you don't point at people for bids. You are working with ladies and gentlemen and your hand should always be an open palm. If you're on TV, there will be different protocol for that too -- you'll need to occasionally address your listeners that are just tuning in. Ramp up your charm, be ever-so-tactful, and wear a bright colored tie or top. In addition to all the prep work you'll have to do (working with clients, working with dealers, etc.) and manning the stand at the auction itself, you'll be sorting out every little detail that occurs that day. And there will be loads! For starters, you'll be working for absentee bidders, handling reserve prices (sometimes the owner doesn't want to go too low -- and even then they might get upset), and working the crowd in between showcases. People will come to you with all their questions, so you have to be prepared with all the answers!
Practice bid calling. Become an entertainer. Get the etiquette down. Know it's more work than what it looks like.