All I done when changing my plugs was be very gentle when withdrawing old plugs.
I used a bit of "blu-tak" in the top of my plug-socket to make sure the plug didn't fall out, it is pretty straght forward.
When putting the new plugs back in, i used the blu-tak again so they didn't drop out, and gently felt for the thread to bite before really screwing them back in.
I also greased the threads before I put the new plugs in.
As for the injectors - i've never done them so can't help

Rowlers