[quote] Drill from the top not the bottom. Start slowly [/quote]
sound advice but once i have drilled the pilot hole i always start from underneath. go in just a PART of the way mid you - say a quarter of the way through maximum. take great care not to go all the way through.
THEN start from the top using the same pilot hole. as you have already part cut through from underneath you avoid break-out and splintering when the hole saw passes through. this can happen a lot when drilling acrylic/fibreglass as the glass fibres tend to snap rather than cut.
just a helpful hint. personally i find it makes for a much neater hole.