Here's a cool little command I never knew about. I was reading Paul Aubins book and he used it. Anyway, I'll make this simple.
Say you have 2 rectangles that overlap in plan view, and you want to represent one as being under the other. Instead of breaking the line, then changing the line style and cleaning up the edges, just use the obscure command. Pick the rectangle you want obscured (the bottom one), right click, aec modify tools, obscure. Then you pick the other rectangle and Viola! Hidden lines and everthing is trimmed nice and neat. Pretty cool command.