I am still working out the secrets to a good monokote/ultracote job, but find that a good jig to hold the pieces securely (especially a big wing) is crucial. This allows you to get one good hand on the edge of the film so you can pull the wrinkles out while kicking the adhesive off with a hot-air gun. Remove the gun and hold the film stretched for a couple seconds while the film cools and sets the adhesive. Works great on rounded wingtips. Get a glove to keep the heat off your stretching hand.

Covering over sheeted balsa, pull the film down tight, heat it, then drop your heat gun and wipe the film down to the balsa while it's still hot. Just a little pressure from a gloved gun hand (soft, nappy glove) will adhere the film with no bubbles and a nice smooth finish to it.

Proper temperature on your iron is also crucial--hot enough to make the adhesive stick, but not so hot you pucker the film. Also, some films (monokote seems to be the worst example) will shrink more along the length of a roll than across the width of the roll. This gets to be a major pain sometimes when I cut a piece along the wrong dimension.