The following advice comes from the official RepRap Wiki:

'With the exception of the hot end and extruder, the pulleys and belts are probably the single most important parts when it comes to improving your print quality [...] To get the best print quality, we want to use a pulley and belt that are designed for linear motion. The GT2 series of belts are designed specifically for that purpose. They use a rounded tooth profile that guarantees that the belt tooth fits smoothly and accurately in the pulley groove, so when you reverse the pulley direction, there is no room for the belt to move in the groove. You should see an almost immediate increase in print consistency, especially around small holes and details'.