If you are designing something so complex that you actually have to think (!?!).. and you get frustrated about how ugly you draw :)
.. or perhaps you'd like to actually be able to rely on your sketches.. I got one word for you: SketchUp!
It is free. It will take a weekend to really get the hang of it. But then it is just your free tool whenever you want a drawing.
It is extremely fast to work with.
You can download tons of pre-mades - see my illustration, the servo I just downloaded.
- And it is (when you have learned the program) very easy to draw in right sizes, so it can be used to actually see if things will fit & work together.
(Please do not ask what the illustrations are supposed to be - it is just part of a brainstorm ;)