“I can’t express what I really want by sketching” that is the most thing students complain to me.
Some are because of the limitation of the sketching capability; others are in the “bottle neck” case (The shape is too complex to represent by sketching). No matter what situation it is, I do suggest making handcrafted model. If you are not good enough to sketch at least you can make shapes by cutting foam with knife. If you are in difficult situation, handcrafted modeling absolutely free for create shapes, and it requirement almost zero skills.
As same as sketching, quantity is necessary, which means you have to pay more time and patients.