Some wisdom that has been bestowed on me
1. Big brush, little paint. - Use the biggest brush you can with as little paint as possible. This gives you clean lines and shadows
2. Washes fix everything - Use washes. Brown for most things, black for whites and bright colours, typhus corrosion (with lots of water) for tanks, oxide (With lots of water) for metals
3. Its not a mistake its a feature. - Occasionally you will find a rough spot where some glue has settled, maybe a dint or a small gap. You can use some liquid green stuff or regular green stuff to hide it or you can make it a feature. darkbrown+Orange dry brushed gives a great corrosion effect.
4. Always do a test model. - This is something I occasionally ignore. But do one, write down what you do, as you do it and put the piece of paper in your paint box or army case. That way you can add new models later with the exact same method.
5. Layer for light colors - White spray has its draw backs, it is very unforgiving. Any slight gap is blaring white instead of a subtle dark shadow. If I'm painting a light colour I use the following - Dark Brown GW base colour, lighter brown colour with bigger brush, bleached bone with a bigger bush, final colour biggest brush that is practical.
6. dry assemble and magnetise - People hate to pull models apart to change them . Magnetising options where you can is going to give you a more flexible army list.