I have found that over-thinking the look is my biggest problem. I use sanded grout because sanded grout is stronger than non-sanded grout. A knife with a serrated blade is great for carving (I also use a rotozip with a drywall bit to carve I'm sure a dremel would do and be eaiser to handle but I don't have one{yet}), a melon baller works great for hollowing areas out. Keep in mind while carving detail, to make it larger as it will fill during the grouting process. I also sand the styrofoam with 60-80 grit sandpaper before grouting, this makes the grout apply MUCH smoother and puts finishing touches on the detail. My wife and I make about 35 pieces a month.
We also use colored grout and apply the temper paints to very damp grout for accents and details. (doing this on 1st and second coats can help you test paint) My projects typically have a coat and a half to two coats of grout with the first being the heaviest coat. Hope this helps.