I have been asked so many times in the last 7 months if I could recommend a place to rent a motorbike or a car for a few days/weeks/months that I considered opening up my own hire place. In the past I have reluctantly pointed them in the direction of the Legacy Gym or Chow Wattana, I say reluctantly because their bikes can be in quite bad shape.
It seems that there is now another and it would seem at first glance better option. It is called Jay-Jay motorcycle hire. Their bikes are fully automatic, 125 cc well maintained and come with a helmet. 250 baht a day and a little cheaper if you require it longer term. They also have a selection of cars available.
Click here for their website.
**Update November 2011** Since first publishing this information I have personally hired a motorbike from this company on three occasions and all three times have been very impressed. Good bikes and excellent service.