Buildup of scar tissue depends on a number of variables. The number of injections you do, the size needle, how straight the needle goes in, whether you move it around too much while injecting, the injection volume, the injection speed, what you're injecting (BA content, crystal size if shooting water based AAS, if the gear crashes out of solution after the injection, etc). Genetics is also a factor.
To minimize buildup of scar tissue, try to keep the injection volume small, and keep the BA content low as possible. Use a fine needle, go straight in, and move it around as little as possible. You should shoot very slowly, taking at the very least 30sec/cc. After the injection, apply uniform pressure to the injection site, but don't massage it. This will spread the oil more evenly with the least amount of inflammation.
Um... I can't think of what else to say, but I'm sure that some of the boys will pick up what I missed.
