My favorite (works with treadmill or bike) is to listen to a soundtrack to an action movie like Charlies Angels. When the music increases it's just natural to increase your speed. During the slower parts increase the incline.
Even better if you're working out at home set up the treadmill in front of an action movie. I find when I bike in front of an action movie my rpms just naturally increase and the sweat starts flying...definitely not the couch potato approach to watching movies. I also find I workout out longer as I tell myself I'll do 30 minutes and before I know it I've biked for the entire movie and my legs are trembling with exhaustion. The key is to remind myself to stretch or I sorely regret it the next day.