As long as you're there by 14:30 (I think that's the cutoff time, but it might be 15:30) they have to serve you that day.
At 16:30 the magic starts to happen... because it's close to quittin time they'll process at about 5x the rate of the earlier hours.
When you get in the taxi say "chaeng wattana soi jet" (soi 7).
Don't forget to bring a pen. They don't supply them. You also need a photo and copies of your passport pic, visa, entry stamps, and i think a copy of the departure card.
Just to confirm as everyone has suggested and provide the most up to date info available, chaeng watthana did NOT ask for a tm-30 today when I did my 30 day tourist extension.