If you close the Ribbon by mistake, you can bring it back by typing Ribbon at the command line. An alternative is to switch workspaces--change to another one, then back to the one with the missing Ribbon.

The Express Tool Extrim (also known as Cookie Cutter Trim) allows you to create a cutting edge from a polyline, line, circle, arc, ellipse, image, or text. Use the selected object to trim multiple objects. In recent releases of AutoCAD (circa 2005 or 2006 and up), it’s accessible only via the command line. Type Extrim at the command line to start the command. Then select the object you want to use as a cutting edge. AutoCAD will prompt you to Specify the side to trim on. Pick a point on the side you want trimmed. The example below shows the results of picking inside the circle, which was selected as the cutting edge. 

Caption: Extrim lets you use the circle as a cutting edge to trim the lines inside the edge.

And this example shows what happens when you pick outside the circle.

Last time we discussed the Qselect (Quick Select) command. In AutoCAD 2011, the Select Similar command provides another way to select objects with similar properties.
With one or more objects selected, right-click to access Select Similar from the shortcut menu that appears.

Caption: Select Similar appears in the right-click shortcut menu when you have an object selected.

Select Similar will select all objects in the drawing that are similar to those you selected initially. To specify the criteria AutoCAD uses to determine just what is similar, type Selectsimilar at the command line, then type SE for Settings.

Caption: Select Similar settings.
There you’ll find a list of properties that you can select. Default settings are Layer and Name. Note that object type (line, polyline, block, etc.) is always a selection criterion.
Name applies only to named objects such as blocks and xrefs.
You can check or uncheck as many boxes as you like to define your selection set. Other options include object properties such as color, linetype, lineweight, and linetype scale. You can also filter based on plot style and object style. Object styles apply to text, mtext, leaders, dimensions, tolerances, tables, and multilines.
If you select more than one object to use with Select Similar, the selection criteria is applied to each object individually. For example, if the Layer filter is enabled and you select two circles, each on a different layer, AutoCAD selects all circles on both layers. If instead you select a circle and a line, AutoCAD selects all the circles on the same layer as the selected circle and all the lines on the same layer as the selected line.
If you aren’t using AutoCAD 2011, you can gain Select Similar functionality via an AutoLISP routine written by Adam Wuellner and posted on the Mistress of the Dorkness blog.

Sometimes when you’re using object snaps, it can be hard to pick the second point. When this happens, try the following tricks:

1. Check to see if there are several objects on top of each other. Zooming in may help you snap onto the correct one.
2. Try typing in the snap command (e.g., Intersection) and then selecting the point.
3. If you’re trying to snap to an object that’s an entity, you may not be able to snap to it.
4. If the object you’re snapping to is in a different elevation plane than the first point, change the elevation so it’s in the same plane.

AutoCAD lets you hide Windows system printers so you don’t have to worry about plotting to the wrong printer. To hide system printers, choose Tools>Options and click the Plotting tab. At the bottom-left corner of the dialog box, check Hide System Printers and click OK.

Caption: Check Hide system printers to remove Windows systems printers from your choices in the Plot dialog box.

Windows system printers won’t appear in the list of plotters displayed in the Plot and Page Setup dialog boxes.

To check the perpendicular distance between parallel lines, type Dist at the command line to start the Distance command. When prompted to Specify first point, type Near, then select a point anywhere on the first line. At the prompt to Specify second point, type Per (for Perpendicular) and then select a point on the second line. AutoCAD will return the shortest distance between the two parallel lines. If the lines are not parallel, use the End object snap override instead of Near when asked to specify the first point. Then select the first line near the end that is closest to the second line.