These are just some general suggestions to make your manga coloring as pleasing to the eye as possible.
Cropping - I suggest you pay attention to what and where you crop. More then likely you are taking a small portion of a whole page when you color. When you do this, try to make sure that no random lines from other panels on the page are in yours. Or more likely than not, your panel may over lap with another. Decide to either include both panels, or if you may need to cut off part of what you are coloring.
Negative Space: ( Definition: Negative space, in art, is the space around and between the subject(s) of an image.) It’s dramatic, draws attention to the parts you really want, and personally I think it looks great when you want a larger canvas. See above for an example of a panel that I made into one with negative space.
2/3rds Rule: This is a little more advanced than cropping and draws upon both the suggestions above to make it work. The 2/3rds rule means that the object in the photo only takes up 2/3s of the frame, and is typically not the center of the image. For an example of what I mean, then click here. So if you crop an image that does not end at the end of the page, aka like her hair in the example link, then add on to the white space to make the frame follow this rule. It’s much more pleasing to the eye, as proven by photographers around the world.