Here are some tips I found useful:
Add a very small amount of vaseline to greenstuff when you mix it: less sticky
Add standard millput to greenstuff to make it harder and easier to carve when set.
Mix GS in advance & let it cure, so its firmer and easier to shape.
Always work in thin layers.
For GS use vaseline to lubricate ur instruments, but also use nailpolish remover to clean/prime the surface of your work after it has set. This will help the next layer adhere. If there are problems with layers not joining use a bit of superglue.
Try Procreate putty, this is like green stuff but less sticky, using more white in your mix gives nice sharp edges.
Try Magiscupt, this is like millput but softer when set so it can be quickly carved. e.g. http://www.coolminiornot.com/134736