As everyone has said- good advice. I started using Vallejo's more this last year. You have to:
1. Clean the figure or part. Sometimes I scuff with a fine scotchbrite pad. You see this technique in some of Verlinden's books.
2. Prime with a good primer that has "tooth". Tamiya Grey is my choice. Let dry overnight.
3. Shake the crap out of the bottle.
4. Apply 2 thin base coats. I use a slightly damp brush, apply one coat, then wait a few hours, and hit it again. Your subsequent color applications will be with dilluted paint (shadows and highlights). We don't want thick house-paint type of coats.
5. When done, the stuff works well with a thin coat of Testors Dullcoat applied with an airbrush. This helps knock out any sheen that pops up.
Although redundant, the above works well for me so far--
Matt