Add lineart, if desired, and erase excess pencil. Make sure your ink is fully dry before beginning!
For cleaner borders, lay down some masking tape
If you leave too much pencil, or don't wait for ink to dry, it will smear and make the marker itself muddy.
This is an avoidable headache!
See above image for an example of too much pencil being left on the page. Erase it until the faintest marks can be seen before proceeding.
Map out your shadows and highlights using a light warm and cool shade.
You can layer them a bit to enhance shadows here and there
Use complimentary tones to make the contrast clearer.
With marker and watercolor, avoid using darker tones early if you intend to layer in lighter colors in those spots afterwards.
A thicker medium like ink or gouache is the only thing that will show up over it without bleeding the colors.
Make sure each layer is dry before adding more.
Add more lineart if desired after all color has dried.
By continuing to use complimentary hues, the piece gains more depth.
Popping some warm tones into a mostly-cool palette also adds depth.
Additional Tips
In lineart, varying line weight can add a lot of dimension to the piece and direct a viewer's eye. Thicker lines stand out more, so (for instance) if your subject is rendered in thinner lines than something in the background, that background element will stand out most.
Don't be afraid to go over some of the lines you've already laid down with a thicker pen (practice is advised, however).
Always ALWAYS test your new markers before you put them to the piece you're working on. Don't rely on the label; a lot of markers are significantly lighter or darker than the color swatch on their lid.
Using multiple colors to fill in something will give it a more lifelike result. Experiment!
If you are using paper:Put another paper or thin cardboard beneath the one you're working on to protect whatever is beneath it. Marker bleeds like nothing else, and if you're working in a sketchbook, you are likely to stain the page beneath your WIP.
Adding highlights with a different medium (gel pen, colored pencil, paint, etc.) can make a huge difference. Try it out!
