The cause of the MAJORITY of wrinkles is trapped air.

1st. As several others have stated, make sure the surfaces are free from any dust, dirt, etc.

2nd. When doing sheeted surfaces, start from the middle and work your way out to the edges WHILE PULLING ON THE COVERING (keep a CONSTANT tension on the covering while ironing it down.

3rd. (IMHO, the most important) On open bay areas AND built up parts, make sure air can circulate between the bays (holes through the ribs insure this) AND, if the part is completely sealed (ailerons, flaps, rudders, elevators, etc), AFTER the part is completely covered drill a 1/16" - 3/32" hole in an inconspicuous spot (normally along the hinge edge ) all the way through to the bay area to allow any build up of air to escape.

This will take care of 99.5% of the wrinkling problems.