In regard to media, you want to look for formats that are both popular as well as unmucked-with.
Images. For example, when archiving images I might choose to keep both the original raw files as well as JPEG copies. With the former I'll hopefully continue to have access to all the data the original image contained. With the latter — because it's such a popular format — there's every chance that I'll be able to view that image decades from now. (If I wanted to be doubly careful I might make an uncompressed TIFF copy of important images.)
Audio. For audio files you can look at AIFF and WAV, which are both popular and uncompressed. MP3 isn't going anywhere anytime soon, but it is a compressed format. Apple Lossless sounds great, but it's a format largely restricted to Apple devices.
Video. When we talk about video, we're thinking of your personal movies rather than commercial releases. (Don't worry, even into the 24th century, Disney will find a way to sell our bunker-dwelling descendants cave-painted copies of The Lion King.)
Check Google for the most common movie file types and you'll find the first hit lists Windows Media, QuickTime, Real Media, MPEG, DIVX, and Flash formats. That may have been a lovely list when it was first put together, but if you know anything about movie files you're chuckling right about now. Some of these formats are all but dead and others will be soon enough.
At this particular point in time MPEG-4 (particularly the H.264 varient) looks like the best bet as it's widely used for streamed and downloaded video as well as for Blu-ray discs. It's typically a lossy format, but high bit rate files can look amazing. Fortunately, you needn't make a decision right now. Keep your original files in the format you prefer — QuickTime, for example. As you see standards change, consider creating copies in the flavor of the decade.
You can't preserve what's not there
Something else to consider is how you back up your data. It's all well and good to create copies of it in forms likely to work on into the next several decades, but if the device or service you've archived it to gives up the ghost a year or two from now, then where are you? It's for this reason that you should have redundant backups of the files and media most precious to you. For now, that means hard drives and online storage. (And hey, what's wrong with printing your images and text?) If you're really serious about this, I urge you to read Rob Griffiths' The Paranoid Person's Guide to a Complete Mac Backup. Implementing his entire workflow is overkill for... well, just about anybody. But it offers some great backup strategies.
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