One trick (I've heard called the Allan Cartography trick, learned from Neil Allan at NACIS some years ago): if you are generating a map for print, and the lines you want to mask out are in one ink color (say black) while the background you don't want to mask (light green, for instance), you can create an outline/halo to the type in 1% black and then set that to overprint. When output, that halo will only knock out the black ink.
Also, I've sometimes had luck making halos semi-transparent: gives the label more prominence, but the halo itself is not so hard-edged.
One thing that may not be clear from previous comments is that for text, there are two ways to get an outline. Simply applying a line to a selected piece of text will apply that line on the character level, where you cannot move it behind the fill in the Appearance pallette. The common habit here is to create a duplicate label and place it behind (that's how we learned to do it in Freehand back int he old days). However, if you go directly to that appearance pallette, and "Add stroke," it adds it to the overall text object, not to the characters, and then you can move the order of fills and strokes, and so place the stroke behind the fill. This then also means you only have one object to move. If you want to apply an effect to a whole bunch of text, you may also have better results grouping them first, or applying the effect to the whole layer.