Carry a notebook with you. No seriously. All the time, everywhere. Make sure at most it's a run to your car to get your notebook. Life is demanding, and you can't always be at a computer. Its not healthy to always be at a computer. But make sure you have a way of writing down ideas you have as soon as they come to you. I have a notebook filled up with personal monologues about the theories behind my game's design, why I chose certain features, the backbones, the structures, the thing my artist needs to do his job. Writing it down will not only help you remember, but it will also give you a better method of processing your ideas instead of just thinking of it once, and trying to program later.
That's a good tip. I started carrying a pocket-sized sketchbook with me a few months back, initially with the intent of practicing drawing to avoid losing the skills I picked up in architecture school. I ended up mostly just writing in it as you described (there's an illustration page here and there, too). It increased my productivity greatly; I remember my ideas more easily if I write them down, and if I don't remember, I can just read them again. I tend to adhere to goals I set for myself better if I write them down, too.
Anyway, thanks for posting this topic, as I'm currently experiencing a total loss of motivation myself and finding these answers useful. Got my XNA-based 2D tile platformer's collision detection working, it's loading and displaying levels correctly, and now I guess I'm bored with it; no desire to work on it at all. I think I'll take a break from this project and read Frank Luna's 3D Game Programming with DirectX11 and Ian Millington's Game Physics Engine Development, which have been sitting on my desk for weeks, taunting me with their fancy math and much-coveted third dimension. There's also the matter of coming up with assets for the 2D game. By the time I'm done with all that, hopefully my interest will have... recharged.