Reddit mentions of Thinkertoys: A Handbook of Creative-Thinking Techniques (2nd Edition)

The Aesthetic of Play:

Play takes place in the mind of the player. The book offers heuristics for analyzing the playfulness of a game. This allows you to create intrinsically engaging games – playful games.

Advanced Game Design: A Systems Approach:

Games are essentially systems – parts that interact with each other in complex ways that generate interesting gameplay. You will learn to observe systems, and start to see them everywhere. You will also learn to create them, harnessing complexity and emergence.

Thinkertoys: A Handbook of Creative-Thinking Techniques:

Not a game design book, but it greatly improved my ability to think creatively, come up with non-obvious solutions, and alter my daily life mindset towards ordinary things. Which is relevant to designing games.

Goal planning depends on your values. Once you can verbalize your values then you'll be able to formulate your goals. That said these resources will help you with the next steps: Read Thinkertoys for an explanation of mind-mapping. GTD by David Allen. Get familiar with Evernote to keep you on track. Watch these for an explanation of how to synthesize the two. Read and watch all the Brian Tracy info you can get your hands on. Become proficient with these resources and you'll accomplish more than you ever dreamed possible

A few more practical techniques (not mine):

Forced association. Attempt to blend the two into something worthwhile. A bad example - lamp and speaker. How about a speaker that looks like a lamp and can be moved around like a flexible lamp arm? This forces the mind to make connections between things that otherwise seem unrelated.

The +-x technique. What can I add/subtract/modify to the design that would make it more creative. Turn it upside down. Completely change the shape. Add something.

I'm explaining this really poorly. Thinker Toys is a fantastic book that gives you practical exercises to help boost creativity and encourage out of the box thinking. I've read a tonne of guides and this one is by far the best.

ThinkerToys - http://www.amazon.com/Thinkertoys-Handbook-Creative-Thinking-Techniques-Edition/dp/1580087736

I took some entrepreneurship courses in college and read "Cracking Creativity" "Your Brain At Work" and "Thinkertoys" as part of the curriculum. I thought all of them were excellent and cracking creativity is especially good for creative thinking

http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Dstripbooks&field-keywords=cracking+creativity&rh=n%3A283155%2Ck%3Acracking+creativity

http://www.amazon.com/Your-Brain-Work-Strategies-Distraction/dp/0061771295/ref=sr_1_1?s=books&ie=UTF8&qid=1449172741&sr=1-1&keywords=your+brain+at+work

http://www.amazon.com/Thinkertoys-Handbook-Creative-Thinking-Techniques-2nd/dp/1580087736/ref=sr_1_1?s=books&ie=UTF8&qid=1449172562&sr=1-1&keywords=thinkertoys

Thinkertoys is a great book on creative thinking.
http://www.amazon.com/Thinkertoys-Handbook-Creative-Thinking-Techniques-Edition/dp/1580087736

In terms of your question in general (i.e. generating useful ideas in general), it's really just a matter of creating a large amount of ideas every day. For me, I have a list of 17 different topics that I generate ideas for (which I pick randomly each day) and then I generate 10 ideas for the random topic that day.

Ignore whether your ideas are "good" or "bad" and just generate them. Oftentimes the best ideas can seem counter-intuitive, unusual, or "bad" at first.

It's also a matter of having the proper mindset as well. For that, I'd recommend listening to "the MFCEO" podcast. Also, reading books related to creativity could also be useful for you. Here's one that helped me a lot:

https://www.amazon.com/Thinkertoys-Handbook-Creative-Thinking-Techniques-2nd/dp/1580087736

Also, don't get stuck waiting for the "perfect" idea, because it's never going to happen and you're going to get stuck overanalyzing things. Just make things and work your ass off.

PM me if you have any more questions! Good luck