Lima is quite a nice city and it can be a marvelous destination if you keep in mind these tips that start when arriving to the airport (located in the city of Callao).

• If you are going to take a taxi to take you to the hotel, there are lots of them available right after you finish all checkings. Just make sure you take a look at the price chart located inside the airport, next to the arriving zone. A taxi to Miraflores, one of the nicest zones in Lima, should not cost you more than $15.0.
• Inside the airport there are also a lot of currency exchange stores. Make sure you get some local currency before you leave ($100.0 is a good start). Currently, $1.0 equals around S/. 3.0 (three soles).
• I know this may be hard to do, but try to change that classic "I'm a tourist with a lot of money to expend" face. People will charge you 5 times for anything you ask if they notice that expression on you.
• Always, and I mean ALWAYS, ask for a discount for anything you get. Be 99.9% sure that you will get it.
• Avoid the following zones unless you go there with someone you fully trust: Downtown Callao, Comas, La Parada, La Victoria, La Perla, San Juan de Lurigancho.
• These are some of the most secure zones in Lima (it would be better to book an hotel that is located in some of these places): Miraflores, San Isidro, San Miguel, San Borja, Barranco, Surco.
• Never leave your personal belongings out of sight when you are in public places. Not even in places like Starbucks, KFC, Pizza Hut, or your hotel lobby.
• If you are a single man/woman looking for fun in a disco and you meet a beautiful local girl/boy, you decide the place to go afterwards. And don't take your eyes out of your drink!
• A taxi to any place should not cost you more than $10.0. A taxi from Miraflores to the airport (a 1 hour ride) usually costs around $7.0 (20 soles).

I hope this tips are useful for you. Your experience in Peru can be marvelous if you manage to follow these few tips.