Suggestions from the RSPB website:
"Half-full plastic bottles or CDs hung up in trees to scare the predators away. Magpies don't like the way light reflects from the surface.
GuardnEyes scarecrow balloon, available from Dazer UK.
It may be possible to deter them by playing a tape of a crow or rook distress call"
http://www.rspb.org.uk/makeahomeforwildlife/advice/gardening/unwantedvisitors/magpies.aspx
However I love magpies; they're stunningly beautiful. If I didn't have a house full of cats I'd be trying to attract them, not repel them!