There are basically two methods that I use prepare work for hanging after the paintings have been completed. I use the same tools but the method differs depending on the size of the piece.
With smaller pieces under 8 inches wide I find that too much hardware on the back makes things feel rather bulky. I have tried sawtooth hangers and eyelet hooks set into the gallery depth canvases but they are awkward for me to work with...maybe it's because my thumbs are double-jointed and don't always cooperate. My favorite method is to use one D-ring at the top center of the canvas. Once it's flipped up, it hooks nicely, hides any hardware used on the wall, can be hung with a small finishing nail, and always hangs straight.
Anything 10 inches and up are considered larger canvases as far as I can see in regards to hanging wires. I measure the canvases and mark 1/3 of the way down the back which is where I place my D-rings on both sides. The thickness of the wire is dependent on the size of the canvas (the wire is labelled by weight) and I typically wrap it around the D-ring twice and then wrap the wire tightly around itself. I don't pull the wire too taught across the back of the canvas so that it's not difficult to hang, but tight enough so that it hangs flat against a wall.