Serious Sam
I urge you to consider creating electrically isolated blocks of track so that you can run 2 trains or run 1 and park 1. See the thread in the Technical section on setting up blocks.
At each power pack, mount 2 terminal blocks (screw barrier terminal blocks are easy to use). Connect 1 terminal on the power pack to 1 terminal block (using 18 gauge wire), and connect the other power pack terminal to the other terminal block. Hook up a jumper (18 gauge is nice here too or Radio Shack has metal bridge strips that do the same thing) to all the screws or lugs on the same side of the terminal block as your power pack connection. The terminal blocks serve as the distribution point for your track power.
For each block, you need a block toggle switch (DPDT center off, at least 12V 1 amp, can be a slide switch but toggles look nicer and are easier to mount). Run a wire (18 gauge) from one terminal block of power pack A to the SAME corner of all the block toggles. Run wires from the other power pack A terminal block to the other corner on the SAME end of the block toggles.
Now run wires from the power pack B terminal blocks to the other set of end terminals on the block toggles, just like you did for power pack A. If you don't have a 2nd power pack yet, you can omit this step until you do.
From each block toggle run an 18 gauge wire from the left center terminal to a terminal block near the block under the layout. Do the same to another terminal block for the right center terminal of the block toggle. These terminal blocks become the distribution points for the track feeders within a block. Again, a jumper is needed to connect all the lugs or screws of the terminal block to the block wires you just ran.
It is important to stay consistent here, wiring the same center terminal to the same rail throughout the layout! On a layout that supposedly runs east and west, one rail, the inner one on a loop, is usually designated the North or N rail. The outer rail of a loop becomes the S rail. So the terminal blocks out near the track blocks should be labeled N or S. And one side of the block toggles becomes the N side and the other the S side. And finally, the same at the power packs - one terminal block is labeled S and one is labeled N. Keep all the S wires separate from the N wires, and only going to their respective S and N terminal blocks.
Finally feeder wires - 24 gauge is good, although some use as heavy as 20 gauge - are soldered to the rail or rail joiner, and the other end connected to the terminal block for the block the feeder is located in. Again, keeping the S and N rail wiring separate and consistent is important!
That's it. It's easier to do than to read or write about.
Hope this helps