So, I'm no expert but here are some tips that I think helped me. Similarly to when you put down two dots and draw straight lines - I will sometimes practice putting down three or four dots and doing curved C or S-lines between these dots, while trying to keep the curve symmetrical, if that makes sense? Then I draw ontop of the initial stroke several times, trying to make it match as best as possible. Maybe that it actually what you were doing aswell but it might not have been clear. Anyways, about your hand being in the way; when I practice these I try to find a good angle of approach (generally I arch from the left and slightly up to the right since I am right-handed - but mixing it up sometimes is good aswell). The angle of approach might help a little but I ultimately I don't think you can avoid the hand being in the way when drawing from certain angles :P Another thing would be "ghosting" your lines (hovering the pencil above but not touching it down while doing the drawing-motion). It may be a good idea to help you with your lines' trajectory. I think I picked this up through the Drawabox-lessons if you are familiar with those? I think what I generally do though when drawing without any aid (as in putting down dots) is I sort of imagine a point on the page instead - or an trajectory if you will, and try to make my lines follow through to the best of my ability. It may sound a bit jumbled, but I hope some of this helps.

If you draw from elbow and shoulder, not resting your hand on the paper, you will have a lot more freedom to keep an eye on your line.