You've already gotten a lot of great advice-- but I think a few more questions are necessary to give you the best/focused solution:

  1. Do you have a dedicated commuter bike?
    • If you do have a dedicated commuter, then you can gear it out-- I'd recommend a front basket that you can drop your bag in. It's easier to balance a bike with the weight centered on the front (I think). And even though it rarely rains in Pheonix, I would recommend permanent fenders for those freak toreential downpours that always seem to happen around the afternoon commute (otherwise you're waiting for the ground to dry).
    • if you use your commuter for mtb-ing or road cycling also like I do, then I think a messenger bag is the way to go... you can drop it low on your back/butt so you don't get a sweaty back (like you probably do with a backpack), and you're not adding a bunch of commuting gear weight for your weekend fun. Timbuk2 still makes the best one, IMO.
  2. Do you shower/change into work clothes, or do you commute in the clothes you work in?
    • I'm guessing based on your question that you shower at work and bring a change of clothes-- which is great except you have to add all that weight to your commute. In this case, a garment bag/pannier on the back of your bike is probably your best option. I personally don't like these because the weight on your bike isn't usually centered and in moments of reflex (like quick stops/turns) it can cause scary results.
    • If you commute in your clothes, then anything that can get your gear off your body is great. See above about a front basket.
  3. Sorry, but commuters don't typically look cool 🙂 ...it's just not in the DNA. With that said you can be less of a dork with these things (in order of importance):
    • Helmet: No one in my hometown under the age of 30 uese a helmet, but for those that do (like me!) it's one of the main things that can separate you from the pack. POC helmets are extermely popular among enthusiasts, though expensive. Nutcase is one of my favs for fun/cool commuting helmets.
    • Lights: A rear red light just below your seat and a front white light on your handlebars... lights in weird places (like on your helmet or backpack) screams "I don't know what I'm doing!"
    • Bike tire pump: ...just don't. Get a CO2 cartridge for emergencies in your commuter bag or saddle bag (under your seat) and avoid the pumps that attach to your frame.
    • Bike Lock: ...same as pump above: don't get something that attaches to your frame. It's super dorky... just put a simple u-lock (heavy, better if you lock outside all day at work) or cable lock (lighter, better if you just park outside for errands) in your bag. Locks just keep honest people honest anyway.

This is probably more than you bargained for in a response, but hey!... you asked 🙂 Good luck with the commuting-- I think it's amazing how many miles we can cover in a week just not driving (not to mention the base fitness that come with it). Feel free to DM me for any explanation on what I said above if it didn't make sense.

Happy riding!