To quickly isolate a layer or layers type in the command line: LAYISO. When the AutoCAD cursor asks, select the layer or layers to be isolated. That will automatically lock all layers except the one you have selected. To restore the layer state just type: LAYUNISO.

To edit or create your own line type:
a)    Type LINETYPE in the command line.
b)    When the Linetype manager dialogue comes up, click on the load button (on top right of the menu).
click on file.
c)    Select the browser arrow down to find out the location of your acad.lin file.
d)    Open notepad or notepad++ and browse for the acad.lin file location.
e)    Follow the format below. Once you are done save the file as a .lin extension.

Line type Definition Format

1- Line type name. (You can use underscore but no spaces).
2- Line type description. (It is limited to 47 characters in length).
3- Line type symbol appearance.
4- Length between each symbol.
5- Drawing unit space length before the symbol.
6- Line type symbol.
7- Type of line type (complex).
8- Line type scale
9- Angle of rotation for the line symbol.
10- X-coordinate position.
11- Y-coordinate position.
12-Drawing unit space length after the symbol.

When you have 3D solids and regions drawings you can flat them to 2D views by using the command FLATSHOT. It will create 2D representations of the current view that you have on your screen at execution of the command. The new 2D entities will be generated as a block projected onto the XY plane. You can always modify the block later since it is comprised of 2D geometry.
That is very helpful, especially when you are working with 3D or dxf files. I know a lot of manufacturers have their online catalog cut drawings in 3D format. So that would save you some time and trouble.

You can use VPMAX  (same as double-click within a viewport layout) or VPMIN (same as double-click outside a viewport layout) to switch within model space and paper space tab of a layout.
VPMAX expands the current viewport for editing and VPMIN restores the current layout viewport.
The advantage of using these two commands is that you can pan and zoom while you are in model space, and when you go back to paper space the position and scale of the objects in the layout viewport remain unchanged.
It is useful when you zoom in a viewport on paper space while your viewport borders are not visible and accidentally double click inside the viewport and you cannot double click out.

Use this command If you need to quickly rotate the crosshairs on the screen at a specific angle so you can draw along any object alignment without having to change The UCS settings nor rotating the view:
In the command line type SNAPANG. When prompted enter the new angle rotation for the crosshairs.

Have you noticed that when you double click inside a viewport in AutoCAD, the view automatically zooms out and then you have to rescale the viewport? It could be really annoying, especially if you do not know the original scale of the viewport. Well, it has something to do with your UCS setting. To correct that, you need to change the UCSFOLLOW value to “0”.  To make the change, try this: while inside the viewport type UCSFOLLOW in the command line and enter “0” for the value. That corrects only the current viewport, because the UCSFOLLOW setting is saved separately for each viewport. Apply the same method for each viewport