A week might be a bit of stretch when storing lettuce, but the very best way is (firstly) to buy the freshest lettuce you can keep your hands on. Crisphead keep better than floppier types like butter lettuce. Then soak the leaves in plenty of really chilly water, drain and spin dry in a salad spinner. Store in a sealed container in the fridge inter-leaved with kitchen paper. This will ensure that the leaves stay as dry as possible.