The last (perhaps first if you're lazy) bit of touch up everyone should do is give their kit a good top coat. Top coats help protect the paint and help reduce the appearance of decals & stickers.

There are three main flavors,

Gloss:

If you're looking for sleek and stylish gloss is what you need. While it can be hard to achieve, mirror finishes can be done. I've never attempted to go that far but I believe the rule is a lot of really thin coats with a very very light sanding in between is how it's done.

Note that too many coats/thick coats can equal an orange peel texture.

Semi-Gloss:

Semi-gloss looks roughly the same as the a Gundam's normal plastic sheen. I've applied several coats of semi-gloss to a model, only to be unable to realize any difference in appearance before and after.

Dull/Flat:

This is what I have started to coat nearly all my kits with. A flat appearance in my mind looks great. Applying a good dull coat over the straight plastic can give the effect of having a coat of flat paint.

Comparison Pictures:

While they're not as good as seeing the results in person these shots should help.

(Note the orange peel texture which begins to show on the second coat)

(once again, note the orange peel texture)

I've only seen top coats come in Lacquer and Acrylic. Are there any differences between them? I haven't encountered any so please speak up if there's something I'm missing.

When it comes to applying a top coat I usually take the easy way out and buy a spray can. You can also buy bottles for brush and airbrush usage. Make sure to read the directions on the bottle to properly thin and apply.

One last note, I've read that a lot of people use Future Floor to give their models a great gloss coat. I've never used it so I can't speak for it personally. (Updated, see above) If you're interested in reading more check out here or here.

I will get some and give it a shot on an up coming project, promise.

Future Update: - I used Future in a comparison test against a spray gloss lacquer. The results are as follow...

Note that there isn't much of a difference between the two. The Future was brushed on, and to my surprise, didn't show brush marks. As with any paint it's easy to apply too much and cause dripping/pooling. I did notice though that it caused streaking from the areas where I applied Gundam Maker. Not sure if I'd have to put down a primer clear coat first to prevent that, but it strikes me as a waste.

Update: WARNING

Do not sure Future in conjunction with Gundam Markers. They will bleed like a mother fucker.