I struggle with this too. Sometimes I'll just let my hand wander around the page making random shapes, then see if I can turn them into something. They almost never look great on the first try, but sometimes I can take something and refine it into something decent.
Another thing to do is keep interesting objects -- any kind of objects -- around your work space. Old tools, bones, wrapping paper, old picture frames, whatever. You may see a shape that inspires and roll with it.
Of course, having reference books to hand is indispensable. I have several books on armor and weapons lying around, and even if I'm not borrowing the designs, it helps to know how the armor actually fits together. Knowing how something works will help you riff off of it.
Nick AvalloneAngryFungus Art Blog