I'm a professional sculptor. There are several other guys here do do sculpting too. HEY WHERE ARE YOU GUYS? GET OVER HERE!
Well I don't know what's become of them.
Anyway, let's see; tips...
There are all sorts of tricks that make sculpting easier.
Keep the green stuff moistened: Although some people say you should lick your tools to keep them moist, this is bad for two reasons:
1) the tiny bit of saliva will dry out in seconds. 2) you will develop CANCER from doing that since green stuff is highly carcinogenic.
Saliva is, however, the best thing for the job. I find the best thing to do is have a little tray filled with liquid. For me this liquid is always composed of two spits worth of drool and about a teaspoon full of hojicha (japanese barley tea, although I'm sure water works too). Mix this up well and if there's any lumps, well, avoid them.
When this is ready you can dip your sculpting tool into the tray and pull out about a tiny half-bead of water. Hover the tool over the work until the bead spreads into a thin - but not soaking wet - layer on it's surface. You can do some of your best sculpting when you get this right. If your tools stick to the work it's counterproductive.
I've also had good luck using a thin layer of Carmex petroleum jelly on large areas of work, but if you need to add any greenstuff after you've applied this it wont stick on properly. With drool water you can just blow on it until it's dry.
Make your own tools: You can do a lot of stuff with needles of various gauges, and off-the-shelf dental tools, but I'd recommend making purpose-built tools for the job at hand more often than not. One thing I do is collect any discarded
lightweight utensils/pencil-like tool handles which can be put into service after wiring or taping a custom tip onto them. These implements should be always be cylindrical so that you can very easily position them with utmost freedom (especially if you have a hook on the end or similar). Square handled tools have a tendency to place themselves in the incorrect position for precision work. One of my favorite tools is a curved needle in an unusually small gauge which I bent carefully and mounted onto the end of my wife's old smart phone stylus which I found in the trash. I drilled a hole in the hard plastic end and mounted the needle into it using green stuff and superglue to secure it fast. The other end still has the old rubber tip, which is brilliantly useful for blending green stuff seams together.
Use a fat rounded grip to hold the work in your left hand: I usually scratchbuild figures onto a wire or ABS plastic frame sticking out of a cylinder of wood which is about 8CM high and 5cm around. But anything in this neighborhood will work as long as it is rounded. When you're holding this with your left hand, you'll find everything flows really smoothly since you can rotate the fig as you work and also (this is important) a fat figure stand keeps your hand from cramping up! You can pick up a ton of them for cheap if you buy a used set of children's wooden blocks.
Collect sets of Dremel bits: Dremel tips have all sorts of things you can use for tiny scale sculpting. Those barrel-shaped heavy gauge sanding tips are great for adding rough texture here and there. Likewise with the radial wire-scour tip used for stripping paint. You can roll that thing around on a fig and get all sorts of neat effects. One of my favorite tips is a micro-grain mini-sander that resembles two lens-shapes stuck together at the edges; the usefulness of this is that when you carefully roll it around on greenstuff it leaves a splendidly bevelled edge which is useful for doing ribbing and trim/piping of all types. There are loads of other tips with all sorts of uses.
Work in layers: Never try to sculpt a whole fig at once. Although Brother Vinni uses sculpey and not green stuff, this 'work and wait' approach is very well illustrated on his 'how to sculpt' page. (you may need to use google translate if you don't speak russian.) The idea is to sculpt a little at a time and allow the work to cure before proceeding so you don't annihilate the detail you just labored to achieve! Also building up thin layers and being patient while things cure is best. You'll find you'll have several projects going at once as you're doing a little sculpting on various models each night. Then sometimes a whole bunch will get done all at once!
http://www.brother-vinni.com/rus/articl ... archer.htm
Clone textures: You can use some of the implements you have collected to do things like this; place a blob of green stuff about the size of a pea or pinhead on the end of a utensil, moisten it with saliva, and push it onto something you've found that has a really neat micro-scale texture. For example, chainmail from an old Ral Partha fig or say, hair from the back of a chewbacca figure (chewbacca is that bigfoot thing from Star Wars.)
Once the green stuff has cured, you can use this to replicate the texture onto your own sculptures. I really wouldn't steal textures from other people's work though. You can use this technique with all sorts of natural objects. Try using an orange to make a lizard-skin stamper!
The most important thing: Be patient!
Keep pushing the green stuff around until it looks right.
If you just settle for imperfect, you'll regret it every time you see the model. Have integrity and make it perfect! It really is way easier to sculpt than it looks. Just persevere. If you've got lots of supplies to play with, don't sit around and waffle. Get your hands moving! You'll be glad you did.
And that reminds me; try not to leave things smooth. It's easier than it seems to give something a cool texture. Collectors really respond to rich textures. In my opinion it's the main key to making a really impressive figure.
Well if I think of anything else I'll post some more...