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.
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).