When creating a science fiction or fantasy illustration, the thing that I find useful, more by far than anything else, is creating rough models and sculptures (maquettes) which I can then arrange into various compositions, photograph from a variety of angles and use to experiment with lighting.
For one of my favorite of my recent small studies of a fantasy subject, this 5x7 inch painting of H. P. Lovecraft's Cthulhu:
I first sculpted this small maquette, about 5 inches tall, to use as visual reference:
As you can see its very rough, very quickly executed (probably took about half an hour to 45 minutes to make) but nothing helps me more in achieving a more realistic, believable effect. I'd be helpless at illustrating these sorts of subjects without this first step in the process.
I'll be starting a larger, more detailed version of this painting this week. It should be up sometime in the next few weeks (though probably not for sale).