Ride with others. Best way, I have found to make you dig deep into your reserves. The example I give is that there is a crossroads near where you live. After 5/10/15/whatever miles you get to this crossroads and ask yourself "Do I go straight on and go home or do I go right and do another 5/10/15/whatever miles?" If you are on your own you don't have the incentive to keep going, so the majority of the time you will head straight on, have a cuppa and cake and fall asleep on the couch. If with other, the majority of the time you will take the second option and throw in a few more miles before home.