How can I post private information such as phone numbers on the Internet, where I can find them later?

Not quite sure what you're asking, as nothing is ever private on the internet BUT:

If you're looking to save stuff to the web in an easily accessible location, get a Google

Docs account, where you can create documents only you can see. (But also have the option of sharing with others)

http://www.drive.google.com/

Do you want to "post", which has the intent of other having access to view if you give them permission, or do you want to "store", where you're the only one (short of being hacked) who has access? For storage purposes, send yourself an email. Update it and forward it to yourself as needed. Keep it in its own folder so you where to find it in the future.

I save some info as a draft email - if that helps

It's too easy to lose a piece of paper, or even your wallet.