Okay, I started the thread so I'll make the first contribution.... Preserving Detail

This may seem unbelievably basic to a lot of you, but I've ruined quite a few models by not following this advice:
NEVER EVER use paint straight from the container. Every coat of paint applied to your tank will remove a small amount of moulding detail, bolts, engine cover recesses, weld seams etc. ALWAYS thin your paint by at least 20% before use if applying by brush.