My best suggestion is trial and error. One of my fav tips though is to use the marker in the opposite direction of your pencil strokes. Or, if you draw in circular scribbles like I do, use the colorless blenders - pencil or marker - in the same manner. Hope this helps, and here's a quick video that shows the use of the Prismacolor colorless marker: [link]
I know, i know you get that a lot, but seriously: i wish i could work/draw like that. It's amazing how real and 3d it looks. The amount of details are just great...the shirt, the hair, even the veins on his arms. i bow and applaud! i have a lot of respect for such talent and work!!!!