Watch the Quarter - Eighth Note Combos online guitar lesson by Brad Carlton from Guitar Lab: Solid Strumming
This lesson combines the quarter notes and the eighth notes to give you a variety of strum patterns. You will receive 16 exercises that demonstrate these various combinations. Adhere to the rule of strict alternate picking for your eighth notes. Once you're comfortable with that approach, then apply consecutive down strokes for eighth notes. These are used for more driving rhythm. I cannot over emphasize the importance of counting every rhythm you play. I guarantee that if you can count it you can play it.
If you're going to be executing a chord change in the next measure, I suggest you use the rhythms that only have a quarter note on beat four. This will give you time to execute the chord change.