Most of the people I know that used to be hardcore IV users and have gone back to sniffing have the same reason: they don't have any more superficial veins that they can hit-or even worse, they have DVT.
For me there is really only one way not to use drugs intravenously, and that is to use drugs that have no benifit when injected (I.e. any Dextroamphetamine formula, benzodiazepines, propylhexadrine, GBL, Buprenorphine IMO is better used rectally, etc..). For instance, when I do finally quit using Needles, it will mean that I will have to never use: Opiates (all of them, because it will just lead back to heroin, Dilaudid, or any of the other ones that provide a rush), Methamphetamine, cocaine, and any other stim known to produce a 'rush' (I prefer stimulant rushes over opiate ones). I have already put an end to my cocaine use, and meth is so sparse here (as well as expensive), that its not a problem for me. I'm basically just trying to cut out the heroin.