Planning and concepting is pretty much the key factor in the arsenal of any game designer, regardless of what area your working in. So long as your creating something, you have to have a clear idea of WHAT you want, HOW your going to do it and WHEN your going to be able to do it by.... its really that simple. However explaining the process is a lot easier than actually doing it, and achieving the ends that you set out to. Every idea starts somewhere, weather its found amongst research, sparked from a doodle on a piece of paper or just an everyday observation, it doesn't matter where it starts so long as the idea creates the inspiration to build upon it and take it further and to an ultimate goal... whether its film, games, music, writing, art in general WHATEVER. The filed you are working in is irrelevant, what is important however is what you do with it.
Now, me personally, I have always been strongest when I'm throwing any random ideas down onto paper via sketching or doodling, whether its an idea from my thoughts or imagination or just inspired from a single word, I have to get the things that particular thing generates in my mind down onto paper to help me better understand whatever the hell it is my brain is concocting. A lot of the time ill do this for awhile, pretty unfocused and I know this is a bad habit, but sometimes ill like an idea visually before ill have the concept built around it. Now what I should ALWAYS do, is as soon as I have an idea I think is strong, begin to look at all the ideas around it building it all up with research, both primary and secondary to make the idea itself founded not just in my imagination, but also in reality... because even if you think of the most fantastical thing imaginable, if it has no reference points or foundation in the real world, its just wont look or feel right to whoever is seeing your creation. A lot of the time, if I struggle for ideas in general research is done first to inspire the first spark that helps get my ideas down onto paper... this is how most people usually work, and It helps in set you on the right foot at the beginning.
Obviously, planning plays a big part of any creative process, so knowing when your going to tackle the different stages of your idea is vital: All areas of design work, whether game or not are all part of a tight schedule, so knowing how your going to do what you intend to do and what the extent of your abilities are in the time given is extremely important. Time management is one of those really tricky things, (that I seem to have yet to master). There are numerous examples of games that have been caught up in time management issues due to lack of foresight in things like development, marketing, changing something at the last minute, lots of things that hit at the worst time possible in a games lifespan.... and also games that because of this, have never seen the light of day at all.
(Research blog looking into the overall game development process)