The most popular way to make your kit look it's best is to paint it's detail lines. Some people think kits look best with out them, others think they look best when done in heavy black. The popular consensus however is that you should line with a color and darkness relating to the plastic you're working on.

For example, when lining on white use a light gray. For medium colors try a darker gray, and dark or somethings vibrant colors use a black. If you're lining with paint try to mix in portions of the base color for a subtle effect.

Remember to try different things and go with what you think looks best.

Lead Pencil:

The easiest way to line is with a lead pencil. Easy to use and fairly easy to clean up (smudging may occur). I've tried it on a kit and wasn't too happy with the outcome. It just didn't look right in my opinion, really hard to explain. Which I why I ended up picking up some...

Gundam Markers:

These babies have been crafted by Bandai to both take more of your money (they're inexpensive, really) and help your Gundam look sexy. I'm not to sure of what sort of ink/paint they use, I heard acrylic, but I can't read Japanese. Gundam markers are just as easy to use as a pencil. When they're still wet they clean up easy with paper towels. When dry you can remove excess with light sandpaper or an eraser.

The two sets I have (seen above mixed up between their original packages) are from the Real Touch line. I'm not sure what makes them special but they work great. One end is a normal tip and the other is fine. Many people recommend fine tipped Sharpie markers but I can't imagine they're as easy to clean up.

For those interested, here's a tip comparison...

From top to bottom, .07mm mechanical pencil, BIC ballpoint pent (courtesy Holiday Inn), fine tipped end then not so fine tipped end of the Real Touch Gundam Marker line.

Eraser:

Great for removing excess pencil and Gundam marker. Get a smudge free one if you can.