Here's a little trick for the TRIM or EXTEND command.  I'll use the TRIM command as an example, but the same steps can be used with the EXTEND command.

After picking a trimming edge, simply pick two points, not objects, with the cursor and the selected objects within or under the crossing window will be trimmed.  You may also hit the ENTER key twice and then use a crossing window to trim with. 

This is one of those rare times in AutoCAD, where picking right to left or left to right will give you the ability to use a  crossing selection.