• always do a "tester" amount first...never a normal dose, just to be sure what you have is heroin and not something else.  This also allows you to judge the purity of the drug
• while many people DON'T do this these days anymore for some reason...go old school and ALWAYS use a filter of some sort.  I had a friend nearly lose an arm because she shot fiberglass into her arm and didn't filter the dope through cotton.
• as much as possible, try to find a reliable dealer who typically has the same cut.  While you can never trust a drug dealer, some are better than others.  I pretty much know what I am getting each time and when my dealer gets a supply from someone he doesn't work with, he always tells me.
• Don't use old needles. A girl I lived with at a halfway house actually had a needle break off in her arm.. it was a couple weeks old and simply fell apart
• If possible, use sterile water....which is next to impossible all the time.  Even bottled water is better than tap though
• Always clean your needles well, even if you don't share them... bacteria can live on the needles and can make you sick
• Always clean the area on your skin where you inject prior to doing so.

Ok.. wow, went off on a tanget there... but who knows.. maybe someone will come across this post and find it useful.