In the spring, for anything that has overwintered, I use a bit of liquid seaweed (maybe as a foliar spray) and some compost with a handful of organic general fertiliser in it. I reckon that gives it a boost without too much nitrogen. Once they begin to form flowers, I try and remember to give them a regular dose of comfrey liquid, or tomato food, which is higher in potash. I get my food from the organic gardening catalogue if that's any help. Oh, my strawberries are in containers, by the way.