You can only enter by land crossing twice in 12 months. But entry by air is unlimited or until IO starts asking questions. This happened to me yesterday, but I just showed my return flight to UK on 31st May, from BKK. I first arrived in Thailand on 26th November.
Many local travel agents will do a border bounce also called visa run, package. Depending on where you are in Thailand average cost is @3000bt. All done in 24 hours.