Clothing - One of the main mistakes is clothing!  Never wear an unbuttoned jacket or longish coat - if it flaps in your dogs face he won't move properly.  Make sure you have nothing that will hinder him.

Height - If you are tall or are handling a puppy, your height is a big disadvantage.  If you use the British style, you dog will end up looking skywards which will spoilt his topline and the position of his head - here seen with Grace:

Standing too close - Spend time with your dog to teach him to stay in position, nothing looks worse than pushing your knee or leg forward to keep him in place or standing too close as seen above.  Remember the judge wants to see your dog and he can't have a good view if you are stood right up to him or have your knee in the way!  This shows how bad this looks with Grace :

Overstretched (or "Rocking Horse" position) - This is where the dogs front legs are too far forward and the back legs too outstretched (although this is sometimes used to describe just rear quarters being overstretched:

Manners - They cost nothing! Don't run your dog up behind someone else's when you are gaiting around the ring.  If you have a dog with good reach, make sure you leave enough space before setting off.  Don't back other exhibitors into a corner with your dog.  If someone does this to you, go to the end of the line and explain to the steward, you have every right to courtesy!