If you use Mac OS X, add the following line to your .bash_profile:
alias passme='openssl rand 48 -base64 | pbcopy'
Whenever you need a password (like when you're registering a new account or resetting your password because yet another online service you used was hacked), just fire up the Terminal and type in
passme. Then paste the password that's copied to your clipboard into the password fields, and into your password manager (I use 1Password).
This alias simply uses openssl to generate a random base64-encoded string with 48 characters (you can change that value to whatever you want). If the online service you use doesn't allow 48 characters in the password field, you should file a support request with that online service, telling them they're being silly only allowing X characters in a password.