Intervenous route technique

I occasionally inject i.v. drugs (from sterile ampules from a trusted hospital source.)  Though I use a sterile syringe/needle each time and swab the area first..etc., every time I shoot I get a big needle bruise at the injection site.  Is there any way to prevent this/attenuate it or should I just find other ways to take my drugs?

I normally use either 30 or 31 gauge needles 8mm.