Luggage and bags: Make sure, you have a bottle of water, some small change and suntan lotion. All 3 are essential. It's blazzing hot from early morning, and the little shop's from which to buy water will probably not have change. The rough guide to Sicily! You will get losed in Palermo! You must have a half decent map on arrival. It is full of essential info on things quintessential to the Palermitani.
Luggage and bags:
If you have nowhere to stay, travel light in the summer - you really don't want to be lugging yourstuff around on a summers day, plus clothes etc are cheap and there are loads of markets..
Clothing/Shoes/Weather Gear: If you are going in the summer remember that in the city alot of people still dress more formally than at the beach - if you go wandering round the cathedral in a bikini not only is it disrespectful but you are bound to get hassled...where possible wear trousers in the city...
Toiletries and Medical Supplies: Sun tan lotion and lots of it, remember you are near Africa - be sure to keep drinking liquids - when the scirocco (wind from North Africa) arrives in mid August it feels rather like how I would imagine to feel if I were a chicken in a fan assisted oven.
Take mossie lotion too - they can be vicious.