I use a messenger bag and rack/panniers.

The stabilizer strap on the bag is really important, at least for me, because the bag always seems to slip around to my front. My wallet, phone, calendars and the laptop (ugh) go in the messenger bag.

Work clothes & shoes and excess folders and books go in the panniers. (We have 3 sites and somedays I have to be in each one...lots of folders and books.) Lunch goes wherever it fits best.

My Jandd panniers have great reflective strips, and zip shut. I just got the "economy" ones and they work well for folders and clothes. I can also run errands on the way home from work with the panniers.

I have seen (Jannd?) panniers that are garment bags, too, but so far my clothes manage to look OK, even suit jackets and skirts. I do pack them in bags inside the panniers for extra water protection (never needed it, even in downpour) and also so that I could leave the pannier on the rack; the hook, rack and fender stays are more than I can take early in the morning!

The one thing I haven't been smart enough to do yet is to take a winter coat to work. That way I wouldn't have to wear my bike jacket with my dressy clothes...

Good luck!