While searching for how people use their screen corners in OS X, I was very surprised to stumble upon a Mac OS X Hints forum comment regarding the standard Apple screen corner triggers available from the Dashboard and Exposé System Preference:

As of 10.4, however, it is also possible to add modifier keys to the hot corners in the Dashboard and Exposé panel of System Preferences. Just hold down Shift, Control, Option, Command (or some combination thereof) while selecting an option from the list.

This may be useful if you find yourself accidentally activating Dashboard, Exposé or even the screen saver when you flick the mouse to a screen corner.

Note that when a modifer key is set, you must have the key pressed before you put the pointer in the corner; it is not enough to hit the modifier key as an afterthought.

I can’t believe I never tried that before. This tip nicely prevents the user from accidentally triggering a screen corner action, which I’ve noticed can happen quite frequency if screen real estate is limited or if the user doesn’t have fantastic control of the mouse/trackpad. Newbie bashing aside, anyone can easily trigger screen corner actions because of Fitts’ law. Your cursor wants to find the corners.

I am trying out the same corner—action assignments I have used for a while now, except now they are triggered by holding down the Command key and moving the mouse into the appropriate corner, instead of just moving the mouse.

Current settings for triggering screen corners, with the use of a modifier key.

There is one potenetial negative to this key-modified corner activation, that is Dual mouse+keyboard use. The benefit of setting a mouse action instead of a keyboard action is so you don’t have to access the keyboard to do something. Now you need to access both simultaneously, hindering casual mouse use.

My other complaint pertains to Apple’s implementation of screen corner actions. Unless the user already knows to hold down said command key and then move the mouse to a corner, user has no way of learning to take advantage of the screen corner actions unless (s)he stumbles upon it while perusing the System Preferences. In contrast both CornerClick and Quicksilver use visual cues to indicate when a screen corner action could be triggered. Quicksilver uses a Airport radar looking cue, while CornerClick has a bubble appear.

Corner Click example (from developer site)

Using Quicksilver to trigger screen corner actions

Alternative settings

Another viable alternative is to use either of these programs to activate screen corner triggers using only the mouse via a corner-click [sic]. I.e. user moves cursor to a screen corner and clicks to trigger the action. This method has the unfortunate drawback of preventing drag-and-drop with screen corner triggering, e.g. when you have done Exposé Show Desktop, started dragging an icon from the Desktop, and want to collapse the open windows back into place using the mouse, or while finding your new email window to drag-and-drop an image onto it.

This snafu can be resolved in Quicksilver by setting up a second trigger for each corner for the same action, using a mouse drag enter as the trigger, instead of the click. In the end, setting up these eight actions is somewhat tedious compared to the Apple solution, so my recommendation is just to stick with that.