Have you tried coloring it. It wouldn't have to be drastic, but sometimes hair color can plump the hair cuticle and make it appear thicker. Also, there are several types of extensions now that are very natural looking. They can be used for volume as well as length. They can be costly, but last for a long time. Also the more product you put on your hair the flatter it will be. The products are heavier than your hair and weight it down. If you use product, use it at the roots only. Try to get your lift  close to the scalp. Learn to backcomb. That can help. Good luck.