Although certain Xperia panels and various plug-ins/themes let you add program shortcuts to the Today screen for quick access to applications and utilities, too many icons can turn to clutter and make finding what you're looking for more difficult.

So I keep my preferred panel clean and rely on the hardware soft keys for launching my most-used apps. To do this, just go to Start -> Settings -> Personal tab -> Buttons. Highlight a key and select the desired action from the "Assign a program" drop-down menu.

If you want to map more than just the soft keys, take a look at X1ButtonMapper