Some tricks that ive heard about is rubbing a small amount of roofing tar onto your rims, thats is supposed to help performance on V - Brakes, i havent tried it though...
Another one when in wet conditions is to cut a few more slots into your braking surface, to drain the water away better, it doesnt effect your overall braking performance, and will improve it in the rain
BTW, Astra, where you from in the UK? Its nice to see another British Rider... Brings the grand total to 4 - 5 actrive british riders on this forum lol