For you low-voltage guys: Is there ever a situation where you've pulled a new wire into a conduit with wire already installed? Is there any way to do this effectively?
See if there's a wire that you can disconnect at one end and use that to pull in a string. Then, pull it, along with the new wire, back with the string.
Also, depending on the circumstances and the kind of wire, you may want to use a pulling lube. Not only makes the pull easier but it reduces the possibility of "burning" the existing cables or conductors.