Gundam markers are probably the most practical way to enhance the look of stock built gundam models. There are three main types available.

The enamel paint injecting varient posses a very fine and narrow tip, allowing the user to enphasise the panel lines by 'drawing' over them. There are three colours available; black, grey and brown. If you want to give your model a realistic touch, then I suggest you reduce the contrast of the panel lines and the surrounding by using combinations:

White or bright colored parts = use grey gundam marker for panel lines

Red or Yellow colored parts = use brown gundam marker

Metalic or dark colored parts = use black gundam marker

If you want to make your model look more anime like, just reverse the above analogy and try to achieve a good amount of contrast between the panel lines and the surrounding color of the area.

If you make a mistake, just use a tissue to wipe the area if it's wet. If the paint becomes dry, wipe the area with a tissue soaked in enamel thinner or use a rubber to rub out the dry enamel paint. But make sure to remove the rubber bits from the plastic part as it damages it over a period of time.

The second variant of the gundam marker is the acrylic paint marker. These don't posses the pointy tips like the enamel injecting ones because these are used to color the actual surface of the parts. By coloring the appropriate areas, you can avoid having to put on those stickers which are included thus allowing for a more advance look overall. These are also a some-what useful alternative to paint brushes as you are not required to mix any paint or thin it down (assuming that the color you want is available in the gundam marker range).

The third variant is the brush type. This is the newest of the three but I've yet to put it through use.

You can utilize these different types to paint even the smallest details, such as the pilot figures which are included with master grade kits.