Oops... And How To Avoid It: Basic Miniature Painting Errors

I had just bought my first box of plastic Dwarfs. I admired the expertly painted miniatures on the cover and thought: How hard could it be? I just need to do a good job. So with this in mind, I dove right in and painted my very first miniatures ever. As in right out of the box with absolutely no clue whatsoever.
The result looked something like a cross between a cheap plastic souvenir and a Kindergarten finger painting. I wasn't thrilled to say the least. But after some thought, I decided to keep the little guy and just keep trying, even though I had no clue as to what I had done wrong or how to get it right.
After a lot of practice, study, and good advice, I finally realized the things I did wrong, and if you remember to avoid them, you can bypass your own versions of the Finger-Painted Model and accelerate your journey to being an excellent painter.
Oops #1: Wrong brush for the job.
Use the right brush for the job. My first miniature was done with a single, cheap general purpose brush, which is no good for things like eyes. If you are going to detail small things like eyes, use a detail brush. One size does not fit all in painting miniatures.
Oops #2: Paint was too thick
This is where you get the Kindergarten look. While the miniatures come with detail, especially the Warhammer miniatures I paint, it will all disappear if your paint is too thick. Fuzzy Dwarf beards suddenly become amorphous facial bibs. Eyes become featureless blanks. Put a layer of paint on anything that's straight out of the tube, and the detail goes away.
This is also responsible for creating a gooey look to paint jobs. Unthinned paint leaves lines in thick streaks which will create a texture on flat surfaces. You can sand these flat surfaces to fix this, but for anything else you're stripping the miniatures and starting over. You can avoid this situation entirely by thinning the paint correctly and remembering that it comes in its tube, jar, or bottle far thicker than it should be applied.
Oops #3: Single layer of paint.
One layer of paint is good for a basic, table grade miniature if you're painting wargaming units quickly. If you want more depth with a minimum of time and fuss you can:
1.) Put down a basecoat and drybrush.
2.) Put down a basecoat, wash with an ink, and then touch up with the same color.
3.) Put down a basecoat layer, a middle layer, and a highlight layer.
But just one layer of paint will make your miniature look flat.
Oops #4: Bottom details first.
Another good way to avoid detail blunders is to paint the bottom details first. If you overpaint when painting an inner detail first, it makes no difference, because the upper surfaces have not yet been painted. Do this with eyes especially, since you can give them razor sharp lines simply by overpainting around them with an upper layer coat.
Oops #5: Shortage of patience.
Painting requires a lot of time and patience. If you just throw on a coat of paint and move on because you want the model done now, you're going to get coverage shortages, and thus a blotchy effect. I've noticed that white, yellow, orange, and red are especially unpleasant in this regard, and tend to require multiple coats.
Again, there are techniques to get decent miniatures quickly, but if you want to paint a competition or display piece, expect to expend a lot of time and patience. There's just no way around it.
Oops #6: Do your homework.
A concept in general that would have helped me avoid some hard knocks with my models. Whenever starting anything new, first do your homework. You won't become an expert without practice, but at least you'll know what to practice, and that will put you well on your way.
These are general things to avoid that will keep you from straying into problems. They will also help minimize mistakes that will leave you wondering why on Earth your subject turned out the way it did.