The following tips were requested by MadMaffMan on the HiveWorldTerra Forum. They are taken from experience of painting a Warhammer 40,000 Ork army, but apply equally well to painting Warhammer models or models from other game systems.
The following painting tips are in no particular order.
My Orks' skin is heavily drybrushed, inked, fairly heavily drybrushed and then drybrushed. It doesn't make it look Golden Demon level blended, but it picks out the detail well and gives them the highlights they need to look good on the table.
Ork clothes are very much the same - get a collection of different browns, paint different models with different combinations and then give it a coat of chestnut ink. This works on most colours from Bubonic Brown through Snakebite Leather to Vermin Brown (and possibly beyond). If you've got a smooth coat of basecoat then the ink just gives you the highlighting and stops it looking so plain (especially bubonic brown)
Part of my reason for drybrushing skin is that it lets you leave a black undercoat in the deepest parts of the musculature. If you then ink over it later, the very deep darkness remains, but gains a slight hint of colour.
A fairly obvious one, but paint in batches of a reasonable number. I find I can usually work on five or six Orks at once. Don't work on too many or you'll find your paint drying out by the end of the batch (especially if it is a mixed colour) and don't work on too few as you'll take longer to complete them as you wait for paints to dry.
Almost counter-intuitive when it is 'quick painting', but don't rush it. The best of paint jobs can be spoiled by a lack of tidyness, and a simple basecoat can still look great if you keep it neat. Always try to get a clean distinction between colours. If you get paint on an unpainted area, paint over it later, and if you get paint on a painted area then either dab/wipe it off with a bit of water and an older (but still soft) paint brush/drybrush or a bit of kitchen roll.
If you just use a small number of paints, you can batch-paint easier and things get much simpler. For example with Space Marines just use chapter colour, chapter colour highlight, black, silver and armour wash for metal areas and one chosen colour for the eye. It won't look quite as detailed as if you pick out all of the details in full colour, but it will look neat, effective and won't turn into a random 'explosion in a paint factory' scheme.
Hive World Terra