Some tips:
1. Sculpt in something that's easy for you to sculpt. Every flaw larger than the thickness of the plastic will show through. You need to get it very nearly perfect, and it's a lot work, so use something that's easy for you to shape even if it's not the best thing to vacuum form over.
2. If what you find easy to sculpt is kinda soft, like oil-based modeling clay, vacuum form thin HIPS (a.k.a. HIS, i.e., high-impact polystyrene) over it and leave it on, to stabilize and protect the surface. (This may not work with water-based clays; I don't use them, but a lot of really excellent sculptors do.)
3. Get the shape as good as you can stand to get it, and then make an intermediate mold. Save that shape, and put it aside in case something happens to the mold or you decide want to improve it and try again. If you can afford it, make the mold with silicone. Look at ways of doing this without wasting a lot of silicone.
(E.g., brush on a thick enough layer of silicone everywhere, and use a mother mold, or build up the shape to the desired silicone thickeness with clay, cast the other half of a two-sided mold from that, scrape off the clay, and pour silicone in between.)
There are people who use silicone caulk for brush-on molds, because you can get it way cheaper than 2-part silicone, but there are tricks to it and I haven't done it well yet.
4. Cast your vacuum forming buck(s) (male molds) in the mold, using something good for the surface layer and something cheap to fill in and strengthen the middle. Vacuum forming exerts 2 to 14 pounds per square inch (depending on whether you're using a vacuum cleaner or a high vacuum system), and can crush a hollow plaster buck, or break a solid one in half if it's not properly supported.
If you break one, you can cast another. You'll often find flaws and fix them on the buck. At that point you may want to make an improved mold from the improved shape. If you break one, you can cast another. You'll often find flaws and fix them on the buck. At that point you may want to make an improved mold from the improved shape.
5. If your shape is not all convex, you will likely have to drill little vent holes through the buck, to let air out of concavities so the plastic can be sucked into them. Plan for this; drilling all the way through big thick bucks is a *****, and eats drill bits. Make your buck in stages, with a relatively thin but reinforced shell, and fill it in but leave little spaces not-too-far inward from the concavities, that you can drill into.
6. Make a sturdy base that's a conservative approximation of what you want out of something cheap. (That is, and undersized, rough version of the shape.) Then use easier-to-work materials to fill that out to the shape you want.
7. Use both additive and subtractive techniques. (That is, glom stuff together when that's easy, cut stuff away if you have to.) Identify the easy parts of shapes and assemble those shapes in pieces, if you can. For example, if you have a straight but rounded edge, use a dowel there as part of your sculpt. If you have a surface that's mostly a simple curve, cut some bulkheads in the shape of the cross-section and bend something flat and smooth around them. Or use a hot-wire foam cutter and cut it out of a block of foam with templates on each end. (RC modelers do that all the time for wings. It works for simple curves, and some compound curves that linearly interpolate between two different cross-sections.)
8. If you're sanding convex curves, make custom concave sanders that are approximately the shapes you need, by cutting a pieces of wood and putting some craft foam and sandpaper over them. Flat sanders suck.
9. For final smoothing, you may want to cast thickish plastic over the buck and leave it there. Unfortunately, if you want sharp edges, you'll round them off a bit when you do that. So you may want to do this to create an intermediate "good shape" plug with smoothed surfaces, then sharpen them up. (You may want to mold and cast after sharpening the edges; if you don't, you'll have to re-sharpen the edges when/if you cast a new buck.)
You can vacuum form EVA foam (like 2 or 3 mm craft foam), and it's great for smoothing things on the scale of about a millimeter. Unfortunately, I don't know where to find it cheap in large sheets. (You can get 12 x 18 craft foam in 2 or 3 mm thicknesses.)
Often, after you smooth away the very small flaws, the medium-scale flaws will be much more obvious; go back and fix them, and do it again.
10. If you're going cheap, and especially if you're vacuum forming directly over your sculpts, don't settle for plaster of Paris. It's just awfully fragile. Use water putty (also available pretty cheap, from the hardware store). Water putty is tougher and sticks to itself better when you re-do an area, and it goes through an intermediate stage where it's pretty workable after setting, before it really hardens up. You can also mix it 50/50 with water putty and get something cheaper than water putty that's much better than plain plaster of Paris.
Reinforce any plaster or plaster-like cement with drywall tape, below the level you might carve/scrape/sand away. (Roughly 1/8" mesh yellow stuff that comes on a roll.) So for example, if casting a buck in a mold, slush on a layer 1/4 to 1/2 thick, lay in some drywall tape, then put in another layer and more drywall tape at a different orientation. (But leave channels near concavities, to drill into.)
11. Have good references, and show your work to people familiar with what you're trying to do, e.g., on the discussion board at tk560.com. They'll see flaws you don't.
Do check out
www.imperial-armor.com and
www.tk560.com, if you haven't yet, for stormtrooper-specific stuff. Also peruse
www.theeffectslab.com, especially the "Sculpture Study" forum. There are people there who really know how to sculpt stuff for molding and casting, and know all about different sculpture media.
If you don't have Thurston James's book "The Prop Builder's Molding and Casting Handbook," buy it or get it from your library. Great book.
