However, I can manage to set (and more importantly keep!) an intention for the length of a class, a day and if I'm really rocking, a week.
The yogic term for this is sankalpa; setting a positive, present-tense intention. I am not going to eat rubbish ever again, becomes I nourish my body with healthy food; I am not going to watch rubbish on TV, becomes I value my free time and make the most of it.
Changing a negative to a positive reminds you every time you repeat your intention that this change is good, it will help you feel better in whatever way, and it may even help those around you as you are healthier, happier or calmer. It doesn't restrict you to always doing the right thing, it sets a general course ahead. If you have a lapse, you haven't failed; life is about balance and you are keeping that in mind. And every time your intention helps you to be better, it strengthens your willpower to keep going.
In January, I will give you the opportunity to set an intention at the beginning of every class. It will be silent and so can be as personal or challenging as you like. And if it is to simply remain present and focussed on your breath and body for the length of your yoga class, that in itself is an achievement.