Don't bring work out clothes. You will not work out. Trust me.
Heels are a waste of space. These cobblestone streets are unforgiving.
It's hot. Like, HOT hot. Bring light shirts, shorts, dresses, and sandals.
Bring approximately 10 outfits. That's all you'll really need.
Food
Siena Kebabs are everything. No, it's not Italian food. And their "kebabs" are essentially burritos. But basically it's the best and you should go all the time.
Take time to explore the city during your first weekend. This way, you'll find the cheapest/best places to get pizza and gelato.
You will miss the diversity of food. It's essentially all bread and cheese here. However, there's a bomb American-style burger place right by the GEO building to give you a little taste of home.
The tomatoes in Italy are inexplicably amazing.
Travel
Everything will probably go wrong (kidding…sorta).
The buses are almost always late, so don't panic.
Said buses may also lack the colors or logo of the bus company, which makes it hard to find. Get there early and ask other bus drivers.
Don't get too bold with your layover times with trains. They get delayed often and missing your train is not particularly great.
Get private hostel rooms. They're essentially hotel rooms, but much cheaper! Also the security of being able to lock your luggage in a room while you explore is priceless.