If you can,a spare battery or a custom higher capacity battery is better.
Those covers etc are popular in iphone since you can't take out the battery on iphone.
+10000 For Android phones I would also go with the spare battery. I always carry 2. They are really cheap on ebay and Amazon as well.
Yeah I already got an extended 3500 mah battery adn I've been using it for quite some time now but I got fed up with the bulky look. I did buy a spare OEM battery but I wanted to ask, I'm definately going to use that second battery every day: I will power down the phone and put the spare battery on it like everyday as I really finish that OEM battery quickly. What I'm trying to say is; would that "on off on off everyday" process harm my phone in any way?