Don’t assume that every vendor is out to overcharge you just because you’re not a local. While bargaining used to be acceptable, it’s a lot less so today. Unless you’re clean sweeping a store, you’re not really expected to shoot numbers back and forth. Obviously, if the price seems ridiculous, do speak out. Loudly. With gestures.
Do tip waiters, taxi drivers, bellboys, and hairdressers. In cafes and restaurants, a 10-15% gratuity is fine. With taxis, you can just round up the amount or add a couple of euros to the fare.