Rotation is raised bed.

As I am limited for space in my garden, my raised beds are constantly being used as my winter brassicas are now planted.

 The question is, how do I prepare the bed with compost and manure ready for the next season if they are being used all the time ? Should I just dig in compost out of the garden bin when I plant my next crop ?

You could try square foot gardening?  This would mean you prepare one square at a time as it becomes available.