You've probably seen serrated plastic knives that are supposed to prevent lettuce and other produce from browning too soon after cutting. Cook's Illustrated finds that these do work, but probably aren't worth the money.
Photo via mealmakeovermoms.
In their experiment, they chopped romaine lettuce and iceberg lettuce using three different knives (one made of stainless steel, another with high-carbon steel, and a special plastic salad knife), and they also tore leaves by hand. Two weeks later, the torn lettuce was the last to brown on its edges -- compared to the 12 days until browning for the heads cut with the metal knives and the 13 days until browning cut with the plastic knife.
Save yourself some drawer space and tear lettuce if you want to prolong its life:
To prolong the life of lettuce by a day or two, stick to tearing by hand. Tearing allows leaves to break along their natural fault lines, rupturing fewer cells and reducing premature browning.
If you're so inclined, however, you can invest in a lettuce knife. WonderHowTo finds that plastic and ceramic knives do slow down browning of other produce such as avocados and apples, so it might be worth the investment.
Lettuce Knives [Cook's Illustrated]