Come up with at least 3-5 new ideas every day. The mind needs to be exercised and trained if you want to keep it in shape. Depending on the kind of things you write, you may only be coming up with ideas every few months and that can mean that you're putting your brain through the same process as trying to get your body in shape by going for a run once a month. You have to use regular conditioning if you want to get in shape, so making yourself come up with 3-5 new ideas every day will ensure that on the days where you do need to come up with something, you're thinking with a brain that's been trained to come up with ideas.

Don't use the first 3 solutions you come up with. If you're trying to come up with a solution to a problem, force yourself not to go with the first three ideas you come up with. Chances are, the first three solutions will be the kinds of things that other writers come up with too, so if you can push beyond those, you're far more likely to come up with a memorable and unique scenario. So if, for example, your character needs to escape from a locked room, your first thought might be to break down the door... which has been done a million times before. Your second thought might be to climb out the window... also, not unique. Your third thought might be to climb through the ventilation ducts... yep, seen that before. And now that you have those all-too-familiar tropes out of the way, you can start to find more creative ways to accomplish the task. Do you pick the lock? Sweet-talk a guard? Signal for help using a broken piece of mirror and the reflection of the sun?

Keep an idea journal. Keep all of your ideas in one place. If you're particularly organized, you might want to keep track of different kinds of ideas (story concepts, plot twists, memorable character moments, etc.) on separate lists. You never know what you'll be writing at some point in the future, so an organized system for keeping track of your ideas will help you go through them later and pull out the good ones that have been waiting around for this exact situation you're writing about.

Periodically re-evaluate the ideas in your journal. If you do keep an idea journal, it's a good idea to go through and re-assess those ideas from time to time. Sometimes an idea that you thought was great when you first wrote it down ends up not being so great when you evaluate it with fresh eyes weeks or months later. And sometimes ideas that aren't quite fully formed need a little more development before they can become the stories they're meant to be. If you make a habit of noting down your ideas, also make a habit of going through those notes from time to time and figuring out which ones you actually think are worth using, and which ones aren't as shiny as they seemed at first.

Get plenty of exercise and do other things. Most of us who have been writing for any amount of time can attest to the fact that ideas seem to come at times when we're doing other things (working out, running errands, right before bed, in the shower, etc.), and very rarely come when we're staring at the blank page or screen. The mind works best when it's being stimulated by other things, not just staring into space trying to will something to happen. Knowing that, make sure you're setting aside time for those other non-writing activities that help foster your creativity. Go outside, be active, and do other things that interest you. Keep a notebook or a voice recorder or your smartphone handy so you can quickly jot down those jolts of inspiration when they strike.

Practice regularly. Put yourself in situations that require you to come up with creative ideas on a regular basis. WdC contests like "The Writer's Cramp - Poetry Week"  and the "Daily Flash Fiction Challenge" , for example, offer a new writing prompt every day. Even if you only enter three rounds per week, that's three days per week you're thinking creatively and trying to come up with a story idea. When I first started getting heavily involved in this site and wanted to develop my writing, I wrote dozens of stories for those contests and the experience of pushing yourself to come up with story ideas on a daily basis for an extended period of time put me in a fertile, creative mindset that's been hard to replicate without the same frequent mental challenge.