Minimum wage in Baja is between $43 and $48 a day. That's not bad you say, well that is in pesos so move the decimal point one to the left. Right around $4 USD per day.
So, many professions count on tipping (propina), which is inflicted on the Mexican customers as well as us gringos. In restaurants you will find 15% still a valued tip. 20% is anticipated in the more upscale/touristy joints. I have found tipping for a haircut, particularly if they own the establishment, surprised them. Here are some additional places visitors to Mexico might not expect to tip:
The gas station attendant: Your attendant should handle all the pump work, clean you windshield, promptly handle you change and factura information if required. $10 (pesos or a buck) on fill seems appreciated. If they handle gas cans, clean them up and place them back in your car, you should throw down a little heavier.
Chamber maids: Now this is expected, but often overlooked in the states. Here the propina makes up a much more significant portion of their wage.
Parking lot security: Many of the larger supermarkets have lot attendants to prevent theft and accidents. (remember you statistically double your chance of having an accident when you put the car in "R") This is particularly helpful if you shop the same place often and toss them $5 or $10 pesos. (fifty cents to a dollar) At first I thought this was a parking lot scam until I checked with the management of Soriana's. Most of these guys you see out there in the 100° heat are pensioners who don't qualify for the minimums. I drop a few coins to these guys when I shop and the other day one of my 'amigos' saw me struggling with my bags, ran over, took my keys and opened my car for me. So far I have yet to get a cart ding in that lot which puts me ahead about $100 USD!
Bag boys (persons) It's a common after school job for kids and now seniors and it doesn't pay squat. A peso or two per bag is appropriate out of the change back from your tab. These kids are working to pay for school uniforms, books and even to support their family at an age when American kids only worry about Playstation and cartoons. Be generous, these kids (or seniors) aren't out spending it on crack!
Wait persons, doormen, bartenders all anticipate and should work for your tip. Don't reward bad service either. Leaving a few pesos rather than none after a meal is still the international sign of distain for bad service as it is in the states. However, if you employ this heavy tactic you might want to back out of the restaurant.
By the way, "codo" means elbow. When used in this context it is often accompanied by tapping the left elbow three times with the right hand. It translate to: "He is such a tightwad he has to be tapped three times in the elbow for the money to come loose from his grip."