Okay, first of all- do NOT tell your insurance company that you're delivering pizza.. your rates would SKY ROCKET. There's a reason that your job title (at least for Domino's/Pizza Hut) does not include the word "driver."
Secondly, you're going to make much more money at night than during the daytime. And like others said, exercise good judgment and familiarize yourself with the area as best as possible.
I do know a couple girls who have been robbed but for the most part, its a safe job if you use common sense (and don't have exceedingly bad luck.)
Obviously be polite and do your best to bring a quick, good product to your customers but a lot of times those things are out of your control.
Tipping will vary and even if you do everything right, sometimes you'll still get stiffed or tipped an insulting amount (IMO $1 is worse than nothing.)
Make sure you have an average car. If its too new/nice, then you're not going to want to put the wear and tear on it. However, you do want something reliable that won't break down on you in the middle of the hood.
I delivered pizza my senior year of college and for the most time, had a great experience. I met some really cool people and got real creative "inventing" different pizzas for myself and my friends to try :)