There's a quick and easy way to create subscripts and superscripts in MTEXT. (Sorry, it doesn't work for single-line text.) While in the MTEXT editor, simply type A^ (for a superscript) or ^A (for a subscript), select the text, and hit the "stack" button (looks like a/b, but...stacked). You can also right-click and select "stack" from the shortcut menu.
Have you ever tried to select a whole bunch of objects, and wondered why their grips didn't show up? The answer is in the "grip object limit", or GRIPOBJLIMIT, which "suppresses the display of grips when the initial selection set includes more than the specified number of objects", according to the help files. The default 100; you can set it higher or lower, depending on your personal preference.
You can quickly add custom shortcut key combinations to your PGP file by going to Tools-->Customize-->Edit Program Parameters. All the way down at the end of the file is a section called “User Defined Command Aliases”. Simply add your shortcuts to that section, following the format of others in the file (shortcut-comma-spaces-asterisk-command – e.g. C, *COPY). Close and save the file, then go back to AutoCAD and type REINIT. Check the box next to PGP file, and click OK. Your new shortcuts are now immediately available for use.
You can control whether AutoCAD goes to the same folder all the time for "look in" or "save as", or whether it remembers where you went the last time you needed a block or an xref. The variable that controls this is REMEMBERFOLDERS. Set it to "0", and AutoCAD uses the "start in" path (from the shorcut's Properties). Set it to "1", and the last used path for each dialog box is saved. This applies across all drawing sessions.
In addition to the standard properties (layer, color, etc.), using MATCHPROP on viewports transfers their visiblity (on/off), locking, and scale. This can be useful if you're aware of it, and suprising if you're not!
In addition to globally controlling layer visibility with freeze/thaw and on/off, you can also control layer display in individual viewports with the "freeze in current viewport" property. (NOTE: You must be in paperspace or in an active paperspace viewport to use this -- it doesn't work in modelspace.) This allows you to show different aspects of the same model in one file, which you might need with demo/new plans, or partial/enlarged plans.
When scaling an object, you have two options for choosing the scale factor: You can enter an absolute number (such as 2, or 1/3), or you can scale by reference. Scaling by reference allows you to scale objects to a desired size without having to calculate the scale factor. To do this, type "R" at the command line after you pick a base point. Next, specify your current measurement, then enter the new distance. For example, if one side of an object is 5'-6" long and you want to expand it to 7'-3", use 5'-6" as the reference length and enter 7'-3" for the new length.
While some commands (like MOVE) automatically end after one action, others (like LINE) will keep going until you tell them to stop. There are four ways to exit these long-running commands: ESC, ENTER, space bar, or a quick right-click.
When using a command that calls for a selection set, like "move" or "erase", you can enter "p" at the command line to retrieve the previous selection set -- the items that were selected by the last command you used.
When you're using the TRIM or EXTEND commands, the objects you're using as cutting or boundary edges don't necessarily have to touch the objects you're trimming/extending. Setting the system variable EDGEMODE to 1 allows you to use an implied intersection as a cutting or boundary edge. This setting will stay until you put it back by setting EDGEMODE to 0.