One of the handy things about getting reminders via Twitter DM is that if you usually get notifications of Twitter DMs via a text on your phone, Growl on Mac OS X, etc. you can get your Habitualist reminders that way too.
To enable Twitter DM as an option for your reminders, just go to Settings then Connections.
Just click on Connect Twitter and you'll be taken to Twitter to confirm that you want to connect.
NOTE: we do NOT tweet on your behalf, either publicly or privately. The connection to Twitter is simply just to prove you own the account you'd like to receive the direct messages on.
The DMs themselves will come from @habitualist so be sure to follow @habitualist otherwise you will not be able to receive our DMs.
Once you've connected Twitter (and followed @habitualist) you can go to any reminder (either general or action-specific) and Twitter Direct Message will be available as a an option to send your reminder to.
Because you can set up as many reminders as you like, and what channel it's sent on is configurable for each reminder, you can do things like get an email at 8am then a Twitter DM at 11am. And, of course, these can be set up on a per-action basis as well so you can target your timing (and nag-level) to each specific action.