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Break it down into as few sticky-outy bits as possible (pedals off cranks etc), tie together as best as you can and then attach to a rucksak - preferably a decent rucksack with a hip belt and back padding.
Done it a couple of times, but if memory serves the main advantage was simply having the folly of youth on my side - it's not comfortable or ideal but definitely doable.
Or as tester says, just push it beside you with a hand on the bars or stem (at least I think that's what he means).
Can you strap a whole bike to a backpack and cycle it home?
Need to collect a fixed beater bike. A frame alone is easy. Assume you can take the bars off and strap the wheels to the frame and make it work?!