I guess the trick is in balance between low relevant keywords and real common needs of ur target audience. It might be smth not so evident so you have to use ur imagination from time to time, e.g . Pabst Blue Ribbon is popular beer among custom motorcycle enthusiasts, vegetarians might be interested in energy-saving equipment, etc. Yes, you must always check the price of ur suggestion, but in fact it's not so time consuming and always helps to lower ur CPC.