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.