Pack Rat
I've started a list of what to pack for my 7 1/2 months in Rome, which span February through September. I always end up packing too much and schlepping something heavy and useless all over Europe every summer. So I'm soliciting advice from you. Name one thing you never regretted dragging with you on vacation/to college/when you moved, and name one thing you totally regretted.

Comments
I've given you my thoughts on this, but...
1. I have never regretting bringing good literary fiction that bears multiple readings, especially if you're not going to be speaking much English during the day.
2. I have regretted bringing too many pairs of shoes, as I generally just end up wearing one or two pairs, exclusively. Then again, I am rarely in fashionable places like Rome.
Avoid bringing anything that needs dry cleaning. Take 2 pairs of shoes max. You will probably buy more there. If you are going to be hiking, participating in digs, etc. bring clothes that you can either toss at the end of your trip or you don't mind losing. Most cities have charity shops so you can always give away the stuff you don't need at the end of your stay.