lessee, what tips that haven't been mentioned...
you could try to make a banner on a computer, print it out, then paint over it-- that'll help you get straighter lines on the banner.
stay away from decals until you can match their detail; they tend to make bad spots seem a little worse in contrast.
you don't have to stay away from highlighting, you just need to use a color much closer to your base colors. mixing paints is a useful talent and easy to learn.
keep your sights low for now; fancy effects like your shield's lightning can come later.
wyrmling_x: yeah, I know about those testors.... My friend tried to do orks with them, and when he tried to paint the teeth it just kinda drooled on him.
Agonofinis: The reason the paint looks so grainy is that I spraypainted him blue. He was the 3rd or 4th mini I did and I did not yet know.