12V wall wart down to 5V
You can't, to have any kind of regulation you need active elements which means transistors. So you will either have to have a regulator, a DC to DC converter (both have transistors) or you can use a resistor divider with limited current. 

Even using a few 5.1 zener diodes (with a series resistor to limit current) is going to use more current than the resistor divider itself (but you get more 'regulation').

So there you have it, now you know why people were so excited when transistors were developed (vacuum tubes needed a 60V or so rail)

Its probably easier to buy a 5V wall wart that has 500mA, they are only a few bucks and it will save you time.