Fertilize the Garlic
Fertilize the garlic with a good nitrogen based fertilizer right when it emerges out of the ground. Pelleted chicken manure, Feathermeal, Chilean Nitrate, or a blended fertilizer are all good organic fertilizers. Stop fertilizing right before the garlic scapes emerge.
Keep the Garlic Weeded
Bulb size is reduced if garlic has to compete with weeds, so get the weeds while they are small. It takes longer to weed when they get bigger. By using mulch, our weed pressure is reduced significantly.
Try to get about 1" of water a week and try to taper off right before harvest. We don't usually ever have to irrigate, since we get "usually" perfect spring rain fall which tapers off into summer storms and then we harvest.