I always seem to have a lot of ideas for personal projects running around my head. Anytime an idea pops into my mind, I write it on a post-it and put it on my bedroom wall. It means I keep thinking about them and helps me work out if they are worth pursuing.
Side projects are incredibly important for improving as a creative. They keep your brain ticking, and teach you about parts of the creative process that are important but often overlooked: time and project management, organisation, payment, production and the dirty (but invaluable) side of design. But they also teach you to trust yourself. You might not have anyone senior to you, or other people around to bounce ideas off, so you learn to value your own opinion and work out what your personal taste is.
Base your project on something you're genuinely passionate about. You'll be more invested in it, and that will push you to produce the best work possible. I co-founded Season Annual (a yearly 400 page publication, now in its second year) with Charlie Sims and Josh Williams. We're all designers with a passion for the beautiful game. And I recently co-created and curate ME & EU with Nathan Smith, a collection of postcards designed by UK-based creatives and sent across Europe in reaction to the referendum result in June 2016.
I know it's an obvious thing to say, but collaboration is key. Find someone to throw an idea at (no matter how crap). It will stop you getting lost in your own thoughts, and can create discussion that might lead to a better idea, or way of realising it. All projects benefit from some kind of external stimulus, so even if you are working alone I seriously recommend having someone who you can talk to, and use as a screening board for your thoughts.