As long as your soil isn't frozen or waterlogged, you can do it now, as well as in the autumn.

Now till the end of March, large clumps can be divided with two forks back to back. Compost and fertilise when you put the plants in their new position.

And if you can't separate clumps with the fork trick described by Docbob above, I'll give you a tip - use a breadknife!

Whichever method you use for the division, if the roots are damaged, dust with sulphur. Also if the ground is wet and cold add sharp sand in the planting holes.