Should be done when its dormant in winter - you can either take it all back to the ground, when you will lose next year's flowers, or cut out last year's stems (meaning 2010) to the ground and reduce the rest by half or two thirds.
In practice, I take off bits whenever I feel like it - if its got out of hand, I remove some of the growth, but I don't hard prune, specially at this time of year.

Thank you!! This is really appreciated, I will certainly be taking your advice :-)