Washes

I use a watercolour wash over a Future base coat.

Apply, allow to dry, usually about 20 mins, then polish off with a kitchen towel.

If you screw it up, you just wipe it down with a peice of dampkitchen towel and start again.

Karl

Matt has a good short tutorial on using sludge washes here...

http://www.swannysmodels.com/Weathering.html

Here's another one from Marc

http://wingnutmodels.com/Washes.html 

I tend to use the flow method where I touch a brush loaded with the wash into panel lines and let capillary action take the wash in. Takes longer, but gives me finer control. In either case, make sure your wash is diluted enough, and your wash is a different medium then your paint. For example, with enamel paint use water based wash with a bit of dishwashing liquicd to break up the surface tension. With acrylic paint, use a oil or enamel wash. I usually give mine paint job a coat of Future - after drying you can use either type of wash.