Tokyo at night after last train

I have lived in Tokyo for almost three months and would like to know whether there is any way to move around the city after 12.00 pm, that is, the time most of the trains (JR or metro) finish. Maybe night buses?
Taxi are so expensive...
Thank you very much.

the majority of the time i was in tokyo i found myself in areas that had no real means of transportation after midnight. buses dont run that late either. theres always a taxi... but as u said its exspensive. i found it cheaper just to wait it out in a bar.. or cafe/karaoke which was my choice if i was trying to save some money ;] (depending on whether your with people) .. and walking is out of the question if u need to take a train more than a few stations away.
i know this isnt much help but i just felt like writing something lol. :]
Good luck.

I assume you mean 12am / midnight (not 12pm in the afternoon) some lines run till 1am, but feeder lines out of central Tokyo.

Pretty much walk. Central Tokyo is very walkable if you get a good map or plan a walking trip using something like google maps. If you have a bike the better, I invested in one of those foldable portable bicycles, yes they are more expensive then your average mama chari (nickname for your average japanese bicycle), I found the portable bicycle great, as you can fold it quickly, bag it and get on a train with one.

Otherwise if you have the will and energy and good planning on maps before hand (again googlemaps is great), Central Tokyo is perfectly walkable, I personally have walked from Tokyo Station to Shinjuku Station before, if I recall it was roughly 2 hours to do so. But I like walking and had no other choice (didn't have my bike with me that day).

Otherwise, yes most of the time if I miss the last train, McDonalds for for the next 4 hours and 100yen menu. I've stayed with friends at Izakaya's overnight as well too. You see many people sleeping.