WARNING: Another long post by me.
Building off of what @Nyran said that's also an incredibly useful technique. So many of my MOCs came about from me literally just holding one piece up to another, or thinking of how I want to use a part.
A good example of "hold a piece up to another one ->Inspiration-> Build->finished MOC" is Nui-Moko, This is also my favorite photo of it. * Giggles *
The inspiration for this MOC smacked me like a fish to the face when I held up the HF 1.0 shin guard piece to the Dragon Bolt head and thought "oh that looks very reptilian, like a dinosaur or alligator, low and behold that's what I built. After the idea struck me and i knew what I wanted, I didn't have a blue shin guard from Surge for the bottom So I thought it would be cool if I kept red on the bottom like the jaw was and blue on top with black as the main structure color since I knew I was going to have to use a lot of HF. in this build.
So how can you do the same you ask? Well first off, you're going to have to fire up your imagination, this technique doesn't really work without it, then just start holding things up to other things see what you get, or look at a part and see what crazy creative off the wall usage of that part you can get, like the Star Wars planet for an eyeball in a Mike Wazouski on Acid(and you're havin' a really bad trip man),
or a Toa Mahri Canister piece as the main body of a futuristic flying two-seater car for Matoran.
These are both MOCs I got the inspiration to build after looking at the part and going "what can I do with this? Other MOCs like Nui-Moko and Dr. Arachnefar
came about by part proximity. The parts by themselves didn't yield anything, but putting them together sparked and ignited inspiration for new MOCs that I had never done before. Then sometimes I just have concepts I'd like to try out and a part I come across simply fits the bill and I go "OOOoo I should use that on this MOC." I never follow one style of building techniques or idea as I find I'm a very fluid MOCer I just go with the flow and I suggest you do the same. Because you never know when its going to be a burst of inspiration right then and there, or a more gradual building up of an idea. Every which way works as long as you're happy with the results.
While using a part as its intended purpose can work(like the dragon bolt mask and accompanying brain piece) it doesn't always have to be a head for a head, or even when you do repurpose parts, try not to be so mundane as chest armor for leg armor or just moving parts around. Get really wild and creative, push boundaries, break mental barriers. If you can do that like I've done, you could create some crazy MOCs that are likely to stand out from the crowd and be memorable.