don't know whereabouts you live, but for those in bigger towns/cities, there are needle exchanges too? as far as rotating sites goes, i am also one of those ppl with less than stellar veins...i fucked up the big veins in the crooks of my arms VERY quickly learning to IV. so for the first two yrs afterward, i could ony hit the veins on the tops of my hands/my fingers/sometimes my wrists. as far as HR goes it was pretty risky. but there are vein maps online that can help you locate and use other sites too...for example i would NOT use my wrists again (veins, arteries and nerves are in waaay too close proximity), but in a pinch I have used my thighs, calves, and forearm veins (LOTS) and those have worked just fine.
as others have said though, the key is rotating your sites....you body will thank you later. also, if you can "save" one or two areas into which you just DO NOT shoot (i.e. your neck, your big wrist vein, etc), that will give you some peace of mind in the event you ever have a medical emergency, and they require a decent spot for an IV.