A lot of people have a fetish for being blackmailed because it allows someone else to have leverage and control over them for a set amount of time. (Perhaps longer. Some may prefer it that way.) The problem is that it may in some cases be too dangerous. I have some alternatives, to the rescue.

1. You hand over control of one of your favorite online accounts to your dominant, and they will give it back to you at the end of the month if you behave well enough. If you don't, they may decide to erase your account completely or just hold onto it. This works by changing the email address and the passwords as well. The password changing could be optional though.
2. Have some really important things on your computer and nowhere else? Add them to a zipped folder with a password on it, and then erase the original files. Give your dom the password. Now only they can give you your files back.
3. Use a remotely controlled padlock, and lock some favorite things inside of a box. Let your dom control the lock. The lock can be broken, but how long does it take for a new lock to arrive? Quite some time, right?

If you have other ideas, send them my way. Always interesting thinking of these things.