Watch the Strum Patterns online guitar lesson by Vicki Genfan from Acoustic Rhythm Survival Guide
Make sure you're getting a real clean sound here. You only want to strike the two strings you want to hear! Experiment with right hand 'palm' muting to get a super tight sound. If your left hand gets tired, please take time out to STRETCH!!!! You will NOT progress if you're holding on to tension that you don't need. For that matter... do a regular 'TENSION CHECK' where you do a body scan and try and find all the muscles in your body that may be holding on to tension that isn't necessary. BREATHING is really important too. Make sure your belly is relaxed.