I find it useful to notice that I am bored or feeling I could be doing something else. Assuming it is something that can wait 20 minutes more often than not I find that noticing it is usually enough. I normally realise it can wait and it is something I notice myself doing more often 'off the mat' too. I feel like I rush through life a lot less now.
The other thing I do is give myself permission to go and do the thing I think I should rather be doing instead of meditating. In the last few years I can think of once I made the choice to get up and even then it was mindfulness in action.
P.S Jon, works with cider too
Years ago when I first tried meditation (before I heard of mindfulness so it was complicated for me by religious overtones) I felt it was impossible and I couldn't do it. The seated pose was painful and I couldn't 'clear my mind' as instructed, it made it more crowded. Mindfulness doesn't expect us to feel any other way than the way we do. It asks us to accept where we are and not judge
It's beautifully simple