You can join corners a lot easier if you have some sort of guide to help you. Most hardware stores carry a few of these corner clamps. For small to medium-sized projects, they’ll definitely save you some time.
Just glue up two pieces of wood along their seams, and clamp one in each arm of the corner clamp to form a perfect 90 degree angle. With four of these clamps you can glue up a box in no time flat, without nails or screws.
The best thing about ’em is they’re cheap. At about $3 a pop the three inch versions pictured here will get the job done. Just be careful not to screw ’em down too tight — you don’t want to leave clamp marks on the wood.