Pattern making tips
The following suggestions are in no particular order (just numbered so that they look like a list)
If you are working in Adobe Photoshop (and Paint Shop Pro from what I can remember) they you can create an image out of layers. Paste the default file at the bottom (the one with the colours on) and on top of that place the primary, secondary, trim, weapon and eye WTP files, removing all of the black from each layer. This allows you to move a layer and check that it lines up with the base layer and all other layers. Once you're done, save each layer as its own TGA with the correct settings.
Use colour overlays
If you use layers of one image, then it can soon all begin to look the same and merge. In Photoshop, add a 25% opacity colour overlay to each layer. If you use a different colour, you can then see what a flat version of the skin (without any dirt) will look like.
Preview in the Army Painter
Use the following lua hack to view all units in the Army Painter:
Extract data\attribacebps\[race name].lua from W40Kdata.sga and modify it with text pad to add the following to the end:
GameData["teamcolour_preview"]["entity_XX"] = "UNIT"
Where XX is the next number in the sequence and UNIT is the name of a unit/pattern (e.g. assault_marine or librarian).
Relic have since changed the format of the Dawn of War files to a faster, compiled binary version. If you want to view more units in the army painter then check out Hangar-8's Army Painter Mods for Dawn of War 1.4 and Dawn of War:Winter Assault or the new UberPainter Army Painter Mod for Dawn of War or UberPainter Army Painter mod for Winter Assault.
Note: this will not show you a perfect representation, as the Army Painter doesn't support partial transparency. Semi-transparent areas are rendered either wholely visible or wholely invisible, depending on whether they are over the threshold.
Start a Skirmish game and save it regularly, aiming to build up an army with one of everything instead of killing the enemy (so it is best to set it on easy). If you then change the WTP files and reload the game, the new patterns will be visible. You will, however, have to keep the same .teamcolour file, badge and banner.
Note: This is the only way to see semi-transparency of textures. Any gradients in transparency on the alpha channel will show correctly in-game.