Hey hey,

Another bunch of hopefully useful advices. I will probably collect it all, make some nice pics and share it as a tut, but don't have a clue when. Probably on my vacation in August.
_{Wait, I was supposed to get some rest then Oh, well.}

Ok, so:

Don't be afraid of experimenting! Use overlay and multiply layers over the one you're working on, with different colors to see how it affects the look of your coloring. Use even dodge and burn sometimes, with different ranges (Highlights, Midtones, Shadows etc), but don't overuse it and don't use it as a main tool of your shading. Rather to highlight some element (works well with metal parts) or to add contrast. Use photo filters. Use adjustment layers. Photoshop is a powerful tool! You can be surprised of the result you wouldn't think of on your own. If you're afraid of spoiling your precious drawing, duplicate layers before experimenting or make a snapshot (the icon in the middle on the bottom of History window in PS. Saves a current state and you can go back to it by clicking the snapshot icon at the top of History window)

Take some rest from time to time. Your brain is tricky and what looks good at the moment (because you've been staring at it for 3 hours), may turn out to be... uhh, wtf? after taking some rest and looking at it with a "fresh eye". It may be also that you hate your picture, and after a break it starts looking quite nice.

Use navigator to see how details affect the whole picture. Working on details, of face, for example, you can easily miss the overall look. Then you zoom it out and...uhh, wtf? Generally, working on a piece on a more detailed level is a constant zooming in and out.

Choose the light source. It will add some 3d to your coloring when some elements will cast shadows. Use some hard and some soft shadows and remember shadows are not black (unless you do an inked piece with shadows included). Shadows may be blue, green, pink, whatever will look good. So do highlights.

Find elements of style you like and try to practice them in your artworks, but don't try to copy "standards". There is no such thing as "standard". It only limits your creativity. Use some realistic looking stuff, search for inspiration in medieval Japanese drawings, in game concept arts... Have fun with experimenting.

Again, study. The more good stuff you watch, the better trained your eye become. You will see things you didn't see earlier. Seriously. It's like discovering a new world.
_{When you also start hearing voices, time to take some rest}

Take your time. If you're not a pro, enjoy this fact Deadlines may be creativity killers. No need to rush when you can do something really outstanding.

Till the next time! And if you have some tips to share, go for it

EDIT: Amazing tut on extracting your lines from the background. A must see! [link]

Part 1: [link]

_{P.S. Sorry for my bad grammar, strange vocabulary etc. Working hard on my English but I'm not a native. I know you'll forgive me}