If you don't change the site once and awhile you'll get a "build up" on the site you use regularly. A build up is tissue that hardens under your skin after awhile. If you alternate, you give the tissue a chance to heal and you won't get the build up. Because I'm so thin I can't use my arms or thighs for injection, so I use the front of my stomach below my belly button, my sides (love handles), and my butt. I use my tummy quite a bit, so I have a bulge there from build up that I'm trying to get rid of right now. What do you use for injecting? - syringes or a pen needle? I find the pen needle is easy to use as you just inject and press the button to deplete the insulin (pen needle is easy for your bum too). Also, when you get build up in a certain area - if you keep using that area, the insulin isn't as effective there, because it cannot go through the build up, so if you're say giving yourself 10 units of insulin, you're not getting the full amount because its stuck in the build up. Hope this has answered your question.