My personal favourite:
1 celery stick (~1 ft long)
1 lebanese cucumber (or 1/2 a continental one)
1 banana
1/2 cup rice milk
Optional (either/or):
* 1 handful baby spinach leaves
* 150g silken tofu (for protein if you're working out).
* 1 tbsp linseed (aka flaxseed) oil (for Omega 3)
* [EDIT] frozen mixed berries are a nice addition.
Throw it all in a blender (maybe chop the celery and cucumber first depending on your blender). Puree. Drink. Doesn't come any easier than that. When I work out in the mornings I have half of it before I go and half when I come back so I'm not too full.
A diet shift like yours is also a lot easier if you have regular small meals, 'cos then you don't get hungry and are less likely to fall back on doughnuts or whatever.