I am a shakey person by default, so when I first started IV heroin, I had a really difficult time with it. Got easier over time, though.
I found that holding my breath worked well. When you need to breathe just exhale slowly and you should be good.
And if you're shooting meth and shakey from that, then you may want to just wait it out until you stop shaking so bad from the meth and just have to contend with your normal shakiness.