Carrots are fussy and they need a deep, fertile soil to do well. Think about the length of a decent carrot and ask yourself the question 'are these seeds going to be able to push down in to my soil and turn in to a nice, long carrot'? If it’s heavy and stony, they probably won’t. So ideally, the soil should be in decent nick up to a spades depth. That means they are one of the only parts of the garden where I do a good bit of work on the soil prior to sowing.
I turn the soil in the carrot bed over with a fork about 3-4 weeks before sowing, so around now is good for me, since I will usually sow my main crop carrots at the end of May. Though this might seem late for a main crop carrot sowing, I find germination far more reliable (and carrot root fly far less a problem) if I wait until then. Also, since I am primarily interested in having carrots as a late autumn / early winter crop to be eaten straight from the soil, I really don’t want them ready too early.
Using the fork I break up any big clumps of soil, and then I leave it settle for a few weeks. Then about a week before sowing, I will get in there and get the bed ready for those tiny carrot seeds. A key mistake that people make when sowing carrots is that they don’t have a fine seed bed, and the carrot seeds end up being too deep or too shallow to germinate. Raking over and back does an amazing job of breaking up the larger clods of soil and getting a really fine tilth. I also add a general purpose organic fertilizer (a handful of seaweed dust per square yard would do the trick).