I'm sure you've all done it. You got an e-mail with a URL in it, so you selected the URL and went over to the Netscape window. Then you wanted to clear out the location field to put this one in, so you selected it and hit Delete. Whoops, you just unselected the URL you wanted to paste! :-)
As it happens, there's an easier way. Once you have the URL you want to paste selected, just click the middle button somewhere in the content area of your Netscape window. (Note: Don't click on a link when you do it.)
If that made no sense to you, perhaps you need to read the tip on pasting in X, which should be appearing in the X11 section here soon.