The trick to having fresh lettuce all summer is to choose your varieties and timing well. You will need to have a succession of different varieties planted every week or so throughout the summer to ensure a constant supply. Start with cold tolerant varieties in the spring, then plant heat- and drought-tolerant ones in the summer, and plants with good holding ability and heat tolerance in late summer and early fall. Plants turn bitter when they are starting to bolt (go to seed) and are exposed to too much heat, so choosing your variety and timing well also avoids bitterness. Here’s an example planting:

May: 'Tango', 'Merveille des Quatres Saisons'
June: 'Dark Red Lollo Rosa', 'Sylvesta', 'Red Saladbowl', ‘Cherokee’
July: 'Reine des Glaces', 'Two Star', 'Red Sails', ‘Romaine Parris Island Cos’
August: 'Reine des Glaces', 'Two Star', 'Red Sails', ‘Mottistone’, ‘Nevada’
September: 'Tango', 'Merveille des Quatres Saisons'