I used a clicker to help my daughter get her chickens comfortable with the showmanship routine. Our chickens love grapes and were quick to catch on that a click meant a tiny piece of grape. Hold out grapes, click the clicker when they come up for one.
So, she'd hold them upside-down, click, hey a grape. Hold the wing, click, grape. They got comfortable in a hurry.
The clicker works great for teaching the walk, also. Touch the chicken behind the leg with a bamboo stick. When it takes a step, click and treat. If the chicken won't take a step, getting her started by luring with the grapes is helpful. I have noticed that chickens who live in more of a free-range environment are much more active and much more willing to walk than a chicken that has been living in a cage. So, if you can get your chicken a protected run, this is a good thing.
There are several books out there about clicker training dogs. It's the same technique regardless of species. I highly recommend "Reaching the Animal Mind" by Karen Pryor as a general overview, but there are probably other clicker training books that are more recipe oriented.
Your chicken will not be judged much on its quality with respect to the standard of perfection, but she will be judged on her condition: good weight, quality of feathers, etc. If you are showing a hen, and she lives with a male, you may have an issue with the rooster damaging the hen's feathers.
Smaller chickens are easier for your to handle, if you have choices. You also want a good mix of docility and activity - not so much that the chicken hops off the table, but enough so she walks.
Wash your chicken sometime during the week before the show.
With feather-footed breeds, you want to keep them out of the mud. With clean-legged breeds, make sure to check for stubs. A little baby oil on legs, comb, and wattles brightens them a bit.
During the class, stand at attention and listen carefully. Make eye contact with the judge frequently. Be ready to answer questions about your chicken and its breed - you should know the varieties, the class, what color eggs it lays, etc. The questions get harder as you move to the upper levels, so the more you read, the better prepared you will be. Backyard Poultry magazine and the Standard are great places to pick up a lot of information quickly and pleasantly.