Here are some of the things I've learned to do when painting and assembling Flames of War miniatures (which are made from either pewter or plastic):
1. Upon opening the blister/box, wash
everything. I close the drain in the sink, fill with water, pour in a little soap, then swirl everything around for a minute. Towel dry. This will get rid of any manufacturing dust that may have remained.
2. Get a good set of hobby clips for getting flash (extra little slivers of material that seeped through the mould seam or overflowed the injection point) and any little remainder from the sprue (the frame pieces are sometimes attached to) off. Files can be used when it's a flat spot, but clips are best to avoid ruining details.
3. Prime, prime, prime! There are 3 colors of primer: black, grey, and white. If you're going for a darker look, use black, lighter: white. Grey is neutral won't change the paint colors terribly much (this is my primer of choice). This includes the bases.
4. When preparing bases, use a hobby knife to make a few cuts on the base before priming. This will help with the glue later on, flat surfaces and mini-model glues don't always get along.
5. Use Elmers or "white/school" glue to glue the minis to a popsicle stick for painting. This provides a nice temporary base for painting purposes, and when you're all done the minis will pop right off.
6. Start with the base color, painting everything in it. Then switch to the next most common color, and work your way up through the detail scale.
7. If you use aerosols for painting (like when priming or topcoating with a clear-coat at the end), don't hose the figures down, instead use short sweeping motions, this will prevent heavy buildup (unless you keep sweeping across the same place every time of course
)
8. Never rush. Rushing causes problems.
9. Try not to mix metallic paints with non-metallics, even with the water cup. Metallics contain little particles that can easily contaminate the nons.
10. Don't paint straight from the paint jar, pour some onto a pallatte first, this will prevent the entire batch from being contaminated. Feel free to water down the paint too, many paints come a little too thick or with too heavy a pigment load to be totally usable right out of the jar.
That's what I can think of so far, sadly for decals you're on your own
Battlefront Miniatures has a pretty good youtube channel where they demonstrate some pretty useful techniques.