Consider the following:
A group of three to six cheeses is a good number of cheeses for a plate.
The cheeses should be arranged in an order progressing from mildest to strongest.
I like to place the first cheese to be tasted (the mildest) at 6 o'clock.
The cheeses should be evenly placed around the plate.
The plate should be large enough to allow for space between each cheese.
The cheeses should fit within the border of the plate.
It is especially effective to alternate textures where possible keeping progression in mind.
A solid colored plate is best and wherever possible introducing some contrasting colors in the cheese selection adds aesthetic appeal.
If you would like to add an accompaniment or a garnish, I would recommend that you place it in the middle of the plate or pass it around on a separate plate.