Watch the Jagged Blues 5 online guitar lesson by Carl Verheyen from S.W.A.T. Blues
Look and listen for a real conscious effort (by me) to lay hard on the 3rds and 7ths of the chords as they change. On the D7 chord I'm hitting the F# and the C a lot. On the G7 chord I play the B and the F, and on the A7 chord I lay on the C# and the G.