Well I've got flowtech shortie headers....i can't push my wire boots all the way onto the plug end, I can feel the metal terminal slide on, and i can push it 75% of the way in, almost all the way, but not enough to hear that "click"

Any tricks or tools to finish it off?

 

Try going at it from under the car

 

use a friend with tiny hands

 

Make sure that the boot isn't sliding off of the wire, sometimes I push the wire out of the boot a little, klick it on then push the boot over the plug.

 

lubricate the inside of the plug cap with some dielectric grease. they will slip right on.