My tips (OK this might be a little bit complicated)
Coloring Large Areas
-Have a picture with no gaps in the line art.
-Use the selection tool to select the area you want to color
-Then go to Select > Modify > Expand
-If your lines are thin, do 1. For thicker lines, put a larger number.
-Go to a new layer and fill in with the bucket tool
-Clean up the corners
Coloring Small Areas
-Set your brush Mode to "Behind"
-If you draw with a new color, it will not cover up the previously used color
-If you do mess up, Lock transparency on the layer by pressing "Square" icon on the Layer panel
-This will let you only draw on top of what is already on the layer
Adding Shading/Highlights
-Create a new layer above the flat color
-Right click on it and select Create Clipping Mask
-It works similar to the Lock Transparency option however you can add multiple layers
-Set the layer Mode to Multiply for darks and Screen for highlights
-You can undo the mask by going back in the the menu or just pulling it away from the layer
Recoloring
-Lock transparency
-Use the bucket tool
-This will not mess up you edges
Quick Color Selection
-Hold down the Option key on a Mac to get a quick dropper tool
-Under the color picker window, if you check "Only Web Colors" you can also get a smaller pallet
-Pressing "X" will quickly switch between your foreground and background color.
Other tips
-B for brush
-E for eraser
-R for rotate
-Number keys for change in opacity. 1 = 10%...5=50%...0=100%
-Use a layer with a solid color which you use to color in your whites, you might discard this layer later or use it as a BG. In order for Clipping Mask to work, you need to fill in your whites