How does bad hair happen? We forget to take the time needed to make it look great! Even if you decide that a length change is needed, sometimes a little texturizing here and there can either keep the Q-tip off the man's head or the bulkiness out of the women's hair.
Hair always seems to grow the fastest where we notice it first - i.e. back of the neck, top of the ear, or the bang area. Hair typically grows about 1/2" a month, so about 6" a year. Regular trims help to avoid split ends and maintain a straight line. When you run your fingers through your hair and have to 'tug' through the ends more than usual, that's when the hair is needing a trim.
Ways to help ensure your hair's healthiness:
Good shampoo and conditioner - maintaining moisture and strength are vital to achieving longer hair. Most over the counter products have alcohol and other drying agents that strip the hair of its natural oils.
Stop washing it every day - I have mentioned this time and time again. The oils our scalp produces adds shine. Don't strip that natural shine out!
Keep thermal styling to a minimum - limit your use of your blowdryer, flat iron, curling iron, etc. Make sure that when you do use thermal tools, you use a thermal protectant. (Aveda's Brilliant hairspray is amazing.)