Don't leave an idea as a voice file. Writing down ideas forces you to actually figure out and understand what you mean. Novice designers are prone to having "ideas" that are only in their heads, and when asked to articulate them, they find out that there's a lot they haven't figured out (or have forgotten).
Aside from the PDA, I have a paper notebook in my "game box," a box where I keep games that I'm play testing. When I'm at game sessions, I can write more extensively in the notebook than I would record on the PDA. (Plus, if I forget one recording device, I'm likely to have on hand the other).
Finally, I have a light laptop/tablet computer, and an even lighter, 700-hours on three AA batteries, solid-state storage, specialized word processor (an Alphasmart Neo) for note-taking when at game meetings. (You can't type on a PDA, not with speed.) I for one don't intend to lose any ideas.
I could keep a voice recorder (such as my Olympus voice recorder) by my bedside for middle-of-the-night ideas, but I don't want to wake my wife with my talking; so I have a clipboard. And I've been known to get up in the middle of the night to write idea details into my computer.
In the 1970s and 80s the "data store" for ideas was notebooks and pieces of paper, sometimes typed (with carbon copies, if you were smart, as a backup). In the 21st century, the data store may still be notebooks, but preferably it is electronic, whether word processing, or a specialist note program such as Info Select or OneNote, or voice messages to yourself. But it's got to be something that can easily be searched electronically and copied (backed up).
Computers are cheap and plentiful. I highly recommend using some kind of free text database. A free text database has no fields such as you define in Microsoft Access or Oracle or (in older days) dBase. You type data in however you like, and the search facility of the free text database does the rest. Any word processor can be used this way, but specialized programs will be faster. Some designers prefer to use a spreadsheet program extensively, but I prefer the superior organization and search-ability of a specialized program.
I have used one of the first free text database programs, called Info Select (www.miclog.com), since the 1980s. It is my "desert isle" program, the one I'd use if I could only have one piece of software. It is fast, easily allows subcategories, and offers many ways to search. (It can also be a word processor, email program, Web browser, etc.) It allows me to not only organize information, but do a full text search in the blink of an eye (because all the stored information is loaded into memory). Unfortunately, it is pretty expensive.
Microsoft One-Note is another program of this type, and it's somewhat expensive unless you're properly associated with a school that subscribes to Microsoft Developers Network Academic Alliance (which makes it free). A very simple free program is Memento, the equivalent of Post-It notes, and there are many other freeware programs.
Or you can use a word processor or spreadsheet and organize your ideas by file. Most computer operating systems allow you to search through files for particular keywords, or the word processor itself may do this. The trick in any of these programs is to have those keywords in the notes you've typed. If you have an idea for a first person shooter, be sure "FPS" is there with the details of your idea. If you have an idea for a card game, be sure "card game" is in the file. Otherwise, when you try to find ideas, you won't find all that you should.
You might think this would take a lot of memory, but it doesn't. An entire novel is roughly one megabyte of text, so as long as you don't store a lot of graphics, it won't put much of a dent in your RAM, let alone your disk space.
If you don't work well from a computer screen, you can print out your ideas and put them in a binder using sheet protectors. Or just have them in a pile. Just be sure to periodically look through your old ideas, as this is one of the best ways to get new ideas.
Another word on images: Storing drawings and pictures results in slower searches because graphics take so much more space than words. Often a program will only search the name of the file, so you need to use long descriptive names. You can use a photo-organizing program such as Picasa (free from Google), or use a database program that handles graphics well.
If you speak to groups, as a teacher or as a proponent of games, be sure to record yourself. An MP3 player with voice recording, such as the Sansa e250, makes this easy to do, and with free software such as Audacity you can turn your talk into a podcast.
In any case, back it up.
All your work will do you no good if your hard drive crashes or you lose a notebook that is your only copy. If ideas are worth generating, they're worth backing up.
Dr. LewisPulsipher comes from the non-electronic side of game design, and teaches video game design at Fayetteville Technical Community College, NC. His most well-known game, Britannia, was described in a recent "Armchair General" online review as "one of the great titles in the world of games."