My Spanish is pretty bad but I will attempt to write what I think is the most important phrase to learn "Lo siento, mi Espanol es malo, hable Ingles" what it means is "I'm sorry, my Spanish is bad, do you speak English?" This has always worked wonders for me. Mexicans realize you are trying to speak their language and showing some respect to them. I usually get a smile and often an answer that their English is bad as well but it's usually good enough to communicate. Don't make the mistake of many arrogant tourists that walk up to a Mexican and immediately start babbling in English assuming they understand it. I find this disrespectful and rude. Imagine someone walking up to you in your country and launching into Chinese or some language you know nothing about.
Mexicans in general are wonderful, generous and friendly people. I've met many that will go out of their way to help you find what you need. Show them the respect they deserve, learn as much Spanish as you can and they will surprise you how helpful they are. The people are one of the big reasons I love Mexico.