Images that are referenced into AutoCAD often show up with a line around their edge – one that you often don’t want to be visible when you print. The variable that controls this border is IMAGEFRAME, and it can be set to 0, 1, or 2. (In AutoCAD 2005 and below, the only options are 0 and 1.)
0: The image border not displayed or plotted
1: The image border is displayed and plotted
2: The image border is displayed, but not plotted
I like setting IMAGEFRAME to 2, because I rarely want images to have borders, but you can’t scale, move, or otherwise manipulate images if you can’t see their edges.
When you need to dimension the distance between two lines that are at an angle, you don't have to use an aligned dimension. Instead, use a linear dimension (just like normal), but before you pick the location of the dimension line, type "R" at the command line. This initiates the "Rotated" option, allowing you to set an angle for your extension lines (the lines that reach out to the object). Voila -- you now have an angled dimension displaying exactly what you were looking for.
You can stretch the command line to display as many (or as few) lines of text as you like. (I keep mine at 2 or 3.) If you need to temporarily see more than that, the F2 key will toggle the display of your recent command line history (the last 400 lines or so). This is useful for many things, including looking up something you typed earlier, or seeing the full results of the LIST command.
You can also undock the command line and move around the screen, resize it, or even close it. (not recommended!)
If you need to use an OSNAP that you don't currently have turned on, you can either type its 3-letter shortcut (such as NEA for Nearest), or use the shift-right-click menu. This lists all the possible osnaps, and even has options for using point filters (where you "inherit" the X or Y coordinate of another point).
The Fillet command now offers the ability to override the current radius to 0 by holding SHIFT while you select the second object. It's convenient because you can quickly make sharp or round corners without changing your radius back and forth.
If you are xref-ing a drawing containing leaders, and want to keep the leaders from plotting, FREEZE the appropriate layer (instead of turning it OFF). It's a bug in AutoCAD that xref-ed leaders on an "off" layer will still plot, even though you can't see them on-screen.
Lines and polylines aren't the only objects that can be used with the FILLET command. You can also fillet between any combination of lines, polylines, arcs, and circles -- although you must use a non-zero radius if you're selecting two arcs or circles. The results are a little unpredictable, but kinda fun to play around with.
To divide an object into an equal number of segments, use the (cleverly named) DIVIDE command. This doesn't actually "break" the object, but instead uses points (or a block of your choice) to mark the intervals. Once this is done, you can either use the points to place objects (like beams framing into a girder), or with the "break at point" command to split up the original object.
Bonus Tip: To be able to see the points after you place them, set PDMODE to a non-zero value. (I like 35, which shows points as a circle with an X through it.)