I've never met a modeler who loved hinging control surfaces. In fact, it's easily the part of constructing a new plane that the average modeler likes least! Although most hinges claim to install as easily as ``just slot the surfaces and add CA'', it's rarely that easy in reality. In fact usually the hinge won't insert properly, or the slot's too loose, or the CA starts to cure while the hinge is not yet in place...and finally you find yourself tugging on the $#%^&! hinge, trying to get it out to start over, with CA on your fingers, and cursing under (or over) your breath. Sound familiar?
Mercifully, there are a few pre-made gapless hinges on the market that seem to be pretty easy to install.
Granite State makes an iron-on hinge that goes the entire length of the control surface. This is a 100% gapless hinge, and requires no CA. It ships in 46'' lengths for about $3 per strip. The color choices are a bit scarce, namely red, while and blue. You can iron Monokote on top of it however.
Fourmost Products makes a unique nylon demountable hinge; there are two nylon segments that you CA to each control surface. A 1/32'' diameter piano wire composes the connecting hinge line. You can readily remove the wire to take your control surfaces apart for building, finishing, or repair. Their most apparent disadvantages: they are heavier than normal hinges, and more visible. They cost $5 for sufficient segments to cover 3 feet of hinge line.
Fourmost Products also makes the Redi-Hinge, a ready-made gapless control surface - two lengths of balsa pre-joined by a polyolefin hinge. Integrate this ready-built hinge into your model, and the hinging's done for you! Disadvantages: this does not seem a likely candidate for retrofitting an old model, and it's not clear how adaptable this ready-made hinge is to any arbitrary model design. They are, as expected, a bit pricier... about $8 for two 2-foot sections.
There are undoubtedly other vendors that market gapless products, but these are ones I have seen myself or used. Feedback on other products that have worked well for you would be appreciated!