YOu have to be sure to believe, abosolutely 100%. If you have any doubt in your mind, no matter how small, you should find what your doubting and fix it, whether it be just talking it out, or by other means.
You should visualize people not paying attention or noticing you. The spell won't stop people noticing you if they are looking for specifcally(sp?) you.
You can use candles and other items to help you focus(personally I don't, because they are to noticable and my family would notive if I lit candles, and I was told that the objects are just there to help focus your mind, and that if yo visualize the objects that they work just as well. That has worked for me so far, though I have only stuck to easier spells, besides once.)
Anyway, sorry for writing so much, hope this helps :)