If you like to work with Lightroom presets, then the just-released OPAL plug-in is a must-have: this plug-in allows you to fade the effect of a Lightroom preset. Without this-plug-in, you'd have to find out what sliders a preset changes and then dial them back individually. This plug-in does it all for you, with one handy master opacity slider.
The video below shows you the plug-in in action. It's available here for $15 (+ VAT in Europe), but code MTW20 will save you a nice 20% ($3) - until further notice. For that price, this is a must-have plug-in for any serious Lightroom user!
Last minute update: the Arrow-Up shortcut only works on Macs. Not sure if this feature will come to Windows computers. For now, Windows users can invoke the plug-in like any other plug-in via File > Plug-in Extras > Opal.