Cook pasta according to package directions, then drain and rinse with cold water.
Add cooled pasta to a large bowl.
Stir in mayonnaise, sour cream, lemon juice, garlic powder, salt, and pepper.
Add bacon and tomatoes and mix well.
Fold in lettuce just before serving to keep it fresh and crisp.
Chill for at least 30 minutes before serving.
Notes
Cool the pasta completely: Warm pasta can make the dressing oily and heavy. Rinsing the pasta with cold water helps stop the cooking and cool it down quickly.
Cook extra bacon: People always want more bacon in BLT Pasta Salad. Making a little extra gives you more to sprinkle on top before serving.
Add lettuce last: Mix the lettuce in right before serving so it stays crunchy and fresh instead of softening in the fridge.
Chill before serving: Letting the salad chill for about 30 minutes helps the dressing coat the pasta better and gives the flavors time to blend together.
Use plenty of salt in the pasta water: Seasoning the pasta while it cooks makes a big difference in the final flavor of the salad.