I put stuff I don't want wet (clothes/sleeping bag) in a trash compactor bag. If rain is a reasonable expectation, I bring a poncho to cover me and my pack and it offers plenty of breath-ability.

Its hard for me to judge what is the correct thing to do about lightning. This link: http://www.lightningsafety.com/nlsi_pls/ploutdoor.htm recommends finding trees of uniform height.