Well here are my tips for picking out good lettuce if your mom doesn't have this in mind:

• Look for heavy lettuce. The heavier the better.
• The least amount of mold (the red stuff) and the least amount of mystery slime at the bottom (that might just be exclusive to Walmart, I dunno) as possible.
• Clean green leaves. All the time. Every time.

Takes some rustling through the entire section of lettuce, but it's worked for us. We chop it, throw it into stainless steel containers, stick it in the fridge...if it's in the back then it lasts us for a couple weeks.