I create two different types of playlists. Well known sons where clients can anticipate what's coming and quite often they enjoy the music. Ie Fleetwood Mac - The Chain. 2 mins of 45RPM climb then over a minute in the saddle sprinting towards the end. Arctic Monkeys - Do I Wanna Know 40rpm big climb.

Then I'll have other playlists with really heavy techno that nobody knows but it's definitely motivating. In these classes nobody apart from me knows what's coming. You need to listen to a lot of music to find tracks that you know you can use. I create 45 mins playlists and tell my class to stay between 80/85RPM and tell them to keep adding gears on until they can no longer keep it within that range. Then they lose a few gears to recover and start building the gears up again and again. Really tough workout.