Patience is number one for hair growth    A good diet and proper care for the hair (so that you don't have to trim damaged hair) will promote good growth.  My experience is that hair grows faster during the warm, summer months if you live in a temperate area.  There are vitamins/supplements that some feel help as well.  I take biotin and silica to minimize hair loss, but I have not noticed that they result in more growth each month.

Good luck with your hair  growth.