Have to agree with Snoopdog... easier if you had dug over if autumn. If it is very wet right now I wouldn't try to dig until it starts to dry out.

Needs lots of organic material added as well - mushroom compost and the like.

Walking on it now will compact it and damage the soil's structure!
When drier, lots of organic material!!
Could also add some fine grit if very heavy clay to improve structure!

All the above is true - the cold weather does breakdown heavy soil if you had dug it over in November. I use a fork, rough organic matter (bark chipping, garden scraps, half decomposted compost and mix it in and dig it over to 2 spade levels deep). I then use good compost for the top few inches. It does lighten the soil for planting and the worms will make it even better over time. Avoid walking on it after wards.

Depends where you are and how bad the winter is as to whether rough digging in autumn breaks it down. This year hasn't really been cold enough for that, and its been far too wet, so most soils currently are claggy and heavy. What Paul the Gardener says about things to add to it I'd back up, and if its possible to get a fork on it and turn it over, do it - that'll help it to dry out a bit.

I did do mine in autumn and it has been fine all winter. I added a lot of sand to it to help it drain better. It seems to have worked well.