When I first started painting (many moons ago) I learned of a technique called black lining. Basically, when you have two light colors right next to one another, the boundary between them tends to get lost, and the mini looses crispness. So, back then, the plan was to put a thin black line between the two colors, which helps them to be distinct.
Now, if you think about it for a minute, you may realize that those same lines dividing your bright colors can be used to divide other colors in the same way. Sure, you may simply want to put your darkest shading at the borders of your color, but if you try it, you'll find that outlining each portion of your mini will work some wonders.
Here is an example of a mini without an outline.
All together, there's nothing wrong with any particular color- all of them have pretty strong contrasts and decent blending. But altogether, this mini doesn't pop- and the reason is because his areas all blend into one another. Now look what happens when we add dividing lines onto the same mini:
Altogether, the lines add a crispness that he was missing without them. Each of the areas is very distinct from the others.
Now, this mini's black lines were added after he was fully painted- and as a result the lines are thicker- giving him a sort of cartoon/comic-book look.
For more realistic looks, try lining as you go. With this technique, it becomes more important that you paint the mini in a proper order.
Think of it as dressing- start with the skin, then go to the next layer of clothing up, and continue from there.
Here is a mini that uses black lines in this way:
The lines on his cloak are probably still thicker than they need to be, but overall, the effect is working quite well.
Among the top painting meta, lining is one of those techniques people just assume you use (really, go check out Marike Reimer's gallery and try to find an area she didn't outline). Note that she doesn't usually use black, but rather uses dark versions of the colors she's lining. Lining with other colors takes a little bit of the edge off of the outlines- it helps the mini look less like a comic and more realistic.
So try lining a mini with dark browns, or using extremely dark versions of the color that you are outlining, or black if you prefer the comic book look. Ultimately, you'll find that all of those little bits on the mini really start to stand out and pop.