We have a double entry visa for two months each, with each entry able to be extended by 30 days. This was our first experience.
i. Don't assume that two months = the same date in two months' time. Check the stamp in your departure card: we arrived on March 21 and assumed that May 20 would be the day to renew, but it was actually May 19. Woops. You don't have to pay the overstay fee of 500 baht per day if you've only overstayed one day though.
ii. Set aside the best part of a day. Some people arrive at 6am, but from what we've heard it's completely unpredictable - you could arrive at 6 and not be seen till 2, or you could arrive at 1 and be in and out within an hour.
iii. We were advised that a friend was deemed "too sexy", so if you're a woman, make sure your shoulders are covered.
iv. Have 1900 baht each on your person.
v. Make sure you have a passport photo with you, and don't forget to bring it as I almost did.
On the day
1. When biking south, make sure you don't get on the flyover section of the freeway, take the exit that says Airport. Otherwise you'll be sent out past the river and that will make you grumpy.
2. Arrive at Chiang Mai Immigration Office, preferably at a time when the entire office is not on its lunch break (we arrived at 12.30). Pick up a form and fill it out.
3. Get your photo and visa pages of your passport photocopied (2 baht/page), sign both pages.
4. Know your address in Thailand.
5. Go to the Information counter to get your number.
7. Meet a nice Thai/Singaporean couple, chat to them about their many businesses and pass the time.
8. Marvel at just how many creepy old white dudes there are.
9. When your number appears on the relevant [ie visa extension] board, hand over your money, form and passport.
10. Sit back down for 20 minutes, worrying that the guy questioning you over your address isn't going to let you stay in the country.
11. When your name is called, pick up your passport and your change stapled to a receipt.
There is a toilet, a cafe selling drinks, water cooler and a photocopying station. There's plenty of seating and the room is air-conditioned, it's all very civilised. All told it took about 3 hours for us and wasn't too painful.