Great answers so far, obviously lots of experience.
The wires are necessary, but there's wire...and then there's wire. Some build 2X4 monstrosities and learn that it's not going to be a two-story structure. 1X2 construction would have sufficed. The same applies to wires. You want to avoid voltage loss, which takes place over distance. 12 gauge is quite a bit too much unless you need a main bus capable of handling 20 amps over 100'. On the huge majority of layouts, 14-16 gauge will be lots, be safe, and be easier to thread through all those small holes you gotta drill.
For the the feeders, I use the wire found in the white flexible plastic sleeve, four of them with red, yellow, green, and black coatings. You can get it by the yard at HD. I think it's 22 gauge, solid copper. It's easily flattened at one end to solder to the rail web or to slide it into a joiner.
I run my benchwork around the walls. So I'm boxed in, and must duck under to enter and exit. That means, when watching trains, I'm always facing a wall. My memory tool is that the two darkest colours for the pairs, red/black and yellow/green, can be kept separate. Red for the closest rail, facing the wall remember, and the darkest wire coating is for the far/closest-to-the-wall rail.