there's the infamous '2 x 20' session, which basically involves working at a constant rate near threshold for 2 x 20 mins with a five-minute rest spinning lightly in between
you can measure 'near-threshold' effort by power or by HR apparently - I measure it on the gym bike by the flow of sweat off the end of my nose
you'll find hods of info if you google it - including this bikeradar thread:
http://www.bikeradar.com/commuting/f...b1e4a712c6b3ac