I keep them in a wiki for now but i think i will change to having a google drive directory for them. More easy to show a google doc to someone or to print it. Six ideas that have enough written on them so it is possible to start to judge them :).

When I was school-aged, I kept it all in notebooks -- never a single, designated notebook, mind you -- just whatever notebook I had on me for class when inspiration (or boredom from class) struck me. I kept, and moved with me through several apartments a large box stuffed with these notebooks that contained game designs, drawings, or other things. Finally I took an afternoon to go through them all and carefully ripped out anything I deemed worth saving. Those are still in a box, albeit a much smaller one. Probably one day I'll digitize them somehow.

When I became older I started recording more of these things digitally.

• I tried Wikis, which are fairly well-suited for these sorts of incremental thought exercises, but the trouble is that you have to administer the damn thing and also the markup language is usually a pain in the ass, along with the built-in entry editor. Probably there's a Wiki package out there that solves those pains, but I never found it when I was going through the wiki phase.
• Microsoft OneNote is pretty good if you have access to it. Its like a wiki, but more WYSIWYG and it has some nice features like full-text search that extends even to being able to OCR text (even handwriting) in images. Alas, its not entirely cross-platform, and its also difficult to track under version control. You maybe don't need that, but its part of the workflow I use for keeping all my important documents safe, and its always nice to be able to find that information you deleted because you didn't think it was important any more.
• Right now my preference is for Emacs Org-mode -- its kind of a text-based One-Note, it allows you to outline documents hierarchically, create multi-level bullet-lists, track progress, and link to other documents and contacts straight from the keyboard. Since its text-based it plays nice with version control, and emacs runs everywhere. You can publish Org mode files as static HTML pages using Jekyll, which is what powers GitHub-Pages, so I can easily share and publish the designs colaboratively with just a github account.

My ideas are separated as several notebooks , few folders, some whiteboards. But hope to organize them soon by getting a Surface like laptop (Pity Microsoft Surface isn't available worldwide but a decent tablet/laptop like Samsung TabPro S or Acer Switch 12 Alpha with 1440p 3:2 IPS screen and pen will solve most of my issues.)