For this healthy smoothie recipe, I tend to use a mix of berries including blackberries, blueberries, raspberries and strawberries with the emphasis on wild blackberries that I've picked that day. I find the blackberries do keep a day or two and any left over, that you are unlikely to use in time, can be frozen.
If you don't like the little seeds from the berries you can strain the smoothie before serving but the seeds are full of goodness too so I tend to leave them in.
I particularly love drinking this as a healthy breakfast smoothie.
If you don't like your smoothies quite so thick add a little skimmed milk.