Some undocumented tricks in AutoCAD can be a real eye opener the first time you realize that it can be done.

Are you still selecting a group of entities with the crossing or all selection option when you Trim or Extend to a lot of entities grouped together in one place?  If you are, then try this simple trick.  After initializing either the Trim or Extend command, issue a return twice.  All the items in the current screen are now selected (without being highlighted) for trimming or extending.  Just begin to pick the entities.  Much faster than grabbing all those entities with a window or using the all selection option.