The best clamps I have a got from Harbor Freight. They are six inch bar clamps, made from plastic except for the metal bar. They have a trigger and rachet for clamping. The "pitch" of this mechanism is very fine, so one can really adjust the pressure just right.
Other than those I also use those spring clamps sold in big bags for cheap price, but those are pretty stiff. I also use wooden clothespins a lot near the tail where I don't need a very wide opening.
I avoid using masking tape across a seam, because glue can wick into the tape and spread to the outside surface of the fuselage, leaving a rough surface.