A tip from Paul Burkhardt
   
   
Some people may know this already but I have come into contact with several people who didn't.
I use the following method a lot and find it very quick and easy when using the trim and extend commands in AutoCAD.

First select the tool button.
Hit hit return or right click once - this does away with the need for selecting an edge to trim/extend to.
Click on the line you wish to trim/extend.

The line will trim/extend to the next intersecting line.

 
If you wish to switch commands or realise you've made an error, hold down the Shift key and this will let you toggle from Trim to Extend or vice versa..