Yesterday's tip talked about applied grayed-out filters when your document mode is Grayscale - but what about CMYK? Rather than switching back and forth between CMYK and RGB, try this: go to the Channels palette and click on the Cyan channel. Now the filters are available, so you just have to apply the same filter to each channel.
Couple of notes: this won't work as well for any filters that generate random effects (such as Clouds) - it's probably best for filters that have settings you can re-use.
To speed things up after you've applied the filter to the first channel, press Command 2, 3 & 4 (PC: Ctrl 2, 3, & 4) to jump to each channel and then Command F (PC: Ctrl F) to reapply the filter using the same settings.
(This tip appears thanks to the option in Blogger to schedule posts ahead of time)