How can you improve? Paint. Keep painting. Paint more. Get a test figure and paint it, strip it, paint it again, over and over. You will discover techniques just by painting.
As for specifics... it sounds like you have some decent tools to work with. Let's see...
Remove all the obvious mold lines.
Make sure you prime your models. It really helps.
Wet palette: yes. Make one yourself, just get a plastic container, a sponge, and some parchment paper. It will keep your paint from drying out.
Depending on your paint, you might need to thin it with water. Multiple thin layers look much better than 1 layer put on thick.
Flow aide/flow improver is very useful stuff.
To make colors pop, a common technique is to paint a lighter color underneath (blue+white), then a darker color on top (blue). This will be brighter than just the end color on top of primer.
Not sure what to tell you on metal. There are many thoughts in this realm. Personally, I paint it black, dry brush metallic silver, and then maybe add some other highlights.
Faces: base color, wash, wash, wash, until you are happy with the pigment.
Eyes: depends on the faction/race. For Orks, I just dot them with red. For humans, you could dot white, then add a tiny dot of black. I've read some people just draw a black line down the middle of human eyes. If they're wearing helmets, you can make the eye holes glow. (Disclosure: I don't paint humans.

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Don't forget to varnish/seal them. Many players will spray with a glossy varnish first (for durability), and then matte after (to take off the shine). Testor's Dullcoate.
Yeah, post some pictures. Hard to give more than general advice if we don't know where you're at right now.
Tons of good info in the Dakka Painting articles section:
http://www.dakkadakka.com/wiki/en/Category
ainting