Future will actually make clear parts gleam like polished gleaming things! It's often used where canopies etc are dipped in the liquid, and then allowed to drain. This gives a perfectly clear, shiny look, better than the original.
It's also used to make transparent parts shine after being sanded and polished to remove scratches, for example.
The Future can be applied by brush or airbrush, over the entire model, before decals, to give an even appearance and smooth surface for the decals to draw down on. Decal solutions are normally in two parts, a softener and a setting solution, the most common being Micro Sol and Micro Set. The Sol softens the decal, after being dipped in water as normal, but before application, allowing the decal to conform over ridges and into recessed detail etc.
The Set is designed to draw down the decal, so that it conforms without silvering, giving a 'painted on' look. This applied to the area to be decalled, and to the decal once in place, and can be quite alarming at first, as the decal my wrinkle and look really strange, until set. Don't touch the decal during this process - it's normal!
Personally, I don't use the 'Set', as it can re-act with Future, and use a small drop of the 'Sol', with some water, in the area of the decal instead. It works just as well.
The method for panel lines described by Glenn is for recessed lines, and will not work the same with raised lines. I normally find that raised panel detail needs very little enhancement, otherwise the whole model looks false. They can be used as a masking guide though, to vary tones and shades, especially on a bare metal finish, and can be highlighted, if desired, by dry-brushing or pencil work.