Here's a couple of my techniques:
1. Photos out of context.
These flowers (above) caught my eye in a decorative garden. If I'd zoomed out, the photo would have depicted a world renowned park in Melbourne. Zoomed in, three's a bit of an odd flora look to these guys. So, how could they be important to the story? Are they dangerous? Contain a cancer containing chemical? Do they only grow on a certain planet? Do they watch you??? The possibilities are endless.
2. Revised history.
Everyone knows about the moon landing in the late 1960's. What would have happened if the astronauts had discovered something unexpected, or brought an infection back to earth with those moon rocks? What about if the US Prohibition had been on something other than alcohol? How would civilisation develop over the next 500 years, if cake, or technology, or cauliflowers had been banned. Thinking that through, its feasible that a cauliflower underground exists on future Earth, especially if future scientists discover some pharmaceutical in the leaves of the cauliflower, or the larvae of the moths that eat them. (I promise never to write the cauliflower underground but you know what I mean).
3. The Time Capsule
This could be for a race, or a town or an individual. You open a package from the past and forever, your life is changed. The package arrives at your isolated space base and it tells you two things - the first is that your parent has died, the second is that in their bedside drawer filled with memorabilia there were two sets of booties, one pink, one blue, two birth certificates, obviously two children. But you're an only child. Where is your sibling? Fast forward 500 years and there are two races at war. What if those siblings, separated at birth, were the founders of those races? How would that differ from them being completely different peoples?
As you can see, there's no shortage of ideas. I'll be writing futuristic for NaNoWriMo, so this is the kind of thing I'll be doing in October.
Do you prep for NaNoWriMo?