Probably the biggest thing I have been taking away from the various guided meditation sessions and discussions I have sat through or listened to has been the principle of "no judgement".
Mind wandering? Fine, just bring it back and don't feel guilty for "letting" it happen.
Need to stop a session before time or maybe even miss one entirely? That's okay, don't get caught up on it.
It's the most wonderful way to sum up what I've been trying to achieve in other areas, especially with regards to the curse of perfection.
Blogging, morning pages, the daily log, anything, if it isn't perfect or doesn't happen the way you want it, or maybe you don't do it at all, that's okay, don't worry, don't judge yourself for it. Just accept it and move on, start again.