For those departing from Phuket: there has been a lot of contention as to whether or not travellers can obtain a re-entry permit from the Phuket Airport. Be safe and get it from the immigration office in Phuket Town instead.
When applying for a re-entry permit you will need to submit the following (you must be flying out of the kingdom on the same day that you apply for a re-entry permit. So get to the airport in plenty of time):
boarding pass (at the airport)
A completed TM-8 form
The correct fee
Colored photo (4cm x 6cm)
Photocopy of passport (main page and latest entry stamp)
photocopies of all of your documents: your passport photo page, your current and valid visa
your TM.6 departure slip
cash to pay for the permit
The current fees for re-entry permits are as follows:
1,000 Baht for single
3,800 Baht for multiple
The fees at the airport may be higher than the fees at the local immigration office. Because this is a last-minute process, you may have an additional administrative fee tacked onto your permit charge. Expect to pay at least 200 THB more.