For my Torpedo Chrome Clearomizer I either turn it upside down every few vapes for the top of the wicks to get a drenching, but I find lifting the top off slightly (not sure if works on all of them) allows more fluid to enter.
I do warn you though be careful if you lift the top part out, firstly make sure the O ring is still in the plastic and is secure, if not using you can push it back down to prevent leaks... I had it in my pocket whilst cycling, just after filling it, the whole top came out! So be wary! :p