IMO, you can dramatically lower the pain associated with Sust injections by following a few simple guidelines. I started off just injecting like Flex described, and when I learned to follow a suggested protocol, it made a huge difference.
1) keep your gear in a dark container in the refriderator
2) place syringe (needle up) in a cup of hot H20 (warming it up to ~100 degrees F is ideal)
3) probe injection area with your index finger to find a "dead spot" that is not as densely filled with nerves/veins as others
4) Make a quick, straight poke and take up to 60 secs to inject the syringe contents. I will sometimes pull the needle out a few mm's during the injection to help spread it out a little.
5) Afterward rub the area thoroughly for about 5 mins.
6) Go do some cardio, or workout shortly afterwards
7) Employ sterile techniques throughout the entire process (drawing gear to cleaning site)
8) Try to combine Sust with another AS like EQ or Deca in the same syringe
It would be very difficult to completely eliminate all of the pain/stiffness associated with sust injections, particularly if your injection volume is high or your injection site is sensitive. However, in my experiences, if you follow these steps, you should be able to avoid the pirate wooden leg walk, and multiple days of pain.