I've taken to using "plumber's tape" to keep wires neat and up against the bottom plywood layer of my layout.
Plumber's tape is not conventional "tape", but rather a semi-hard plastic tape about 1" wide and 1/8" thick that plumbers use to hold pipes in place. It's the modern version of the old copper strips with holes that plumbers use.
The tape comes in rolls of 100' or so. Basically, depending on how many wires I need to staple in place and how thick they are, I cut about a 3"- 4" long piece of the tape off the roll; place it over the wire(s) so that an equal amount of the tape extends out about 1" on either side of the wire(s); and then just staple the tape ends to the plywood with a heavy duty or electric stapler using 3/16" to 1/4" heavy duty staples.
The tape firmly holds the wire against the plywood and, to me anyway, is easier than using cable-type clamps. You can even bend the tape in the middle to form a little "loop" for the wire(s) to sit in.
It's available in white or gray on the big A or at most hardware/big box stores.
