My Ortho All About Pruning book says that late winter is the best time to do heavy pruning on an yew. But that's about all it says.
I think (and I'm sure I'll be corrected if I'm wrong)
that the general rule of thumb is to take care not to remove more than a third of the branches per year. So if you have a lot to take off, you'll have to do it in installments of a third at a time. So if your yew has 15 branches, you can take off five this year. Take each branch off all the way to it's collar on the trunk so it can scab over easily. With a yew, this means wearing long sleeves and gloves and reaching into the thing to find where the the branch collar is. Take off the crummiest-looking branches first, or the ones that are most disruptive to the natural shape of the shrub.
I hope this info is helpful to you.
