Make sure you get a good rack. Do not go for a seatpost rack, as they cannot handle heavier weights (and they shift around).

Get good-quality panniers. I made the mistake of getting some Nashbar grocery panniers, and all my groceries fell out the first (and only) time I used them because they were not secure.

For my commuting, I use an Arkel briefcase pannier for my laptop when I need to haul it around. Otherwise, I use some cheaper Axiom panniers. They work fine for my clothes, but are too small for many groceries.

I really like Arkel panniers (www.arkel-designs.com). They are well-made, made in the US and Canada, and have a very secure attachment. They also have a variety of styles. They are not cheap, but they will last and last, and they will stay on your bike (very important).