There are a lot of shortcuts.
Here are some that I've used in the past;
- Using coloured primer. Check out armypainter.com - they've got a selection of coloured spray primers. These are actual primers, but come in the base color of your army.
- Use foundation paints - they cover better, so you don't need as many coats.
- Use an airbrush - either for basecoating, or for shading/highlighting.
- Paint in an assembly-line style. If you're going to do 10 bolter dudes, do each step on each model then move on to the next step.
- Use larger brushes. I love my #2 filbert brush from Princeton Art&Brush co.
It is a wide brush that's great for covering areas fast, but it tapers to a rounded point, allowing precision as well. It's also a flat brush, rather than a round brush, so you can use the thin side for even more precision.
The bigger the brush, the less frequently you need to dip it, and the more surface area you cover per stroke. Of course, you need to really master brush control but it speeds a lot of things up.
- Cheat! Rather than worry about minor mistakes, use them as the basis for combat damage or weathering. It's faster to dab some black over your mistake and then hit it with a metallic point than it is to carefully match the colour. This only works for armour... but mud on pants legs, or blood on flesh might work too...