From eHow: How to harvest lettuce leaves
Leaf lettuce produces clusters of loose leaves instead of the tight heads typical of head lettuce varieties, such as iceberg and buttercrisp types. The leaves do not surround a dense heart, so you don’t have to harvest the whole plant at once. Harvesting instead takes place over the course of several weeks or months, which allows the lettuce to continue growing and producing for a longer time than head lettuce varieties.
1. Cut off the outer leaves when they are at least 2 inches long. Remove the leaves at the base of the plant but leave the inner foliage in place to continue producing.
2. Harvest the outer leaves every three to five days during the growing season. Picking the lettuce more frequently may prevent the plant from producing more foliage.
3. Harvest the entire plant before it begins to flower, usually when the average daytime temperature reaches 70 to 75 degrees Fahrenheit. Cut off the plant at its base, harvesting all the leaves at once.
eHow also has another article on how to harvest lettuce so the leaves keep growing. Most interesting point: Do not tear leaves; cut them instead. Read the article for why.