Some time ago I discovered a nice (and easy!) tip to decrease the size of a huge Photoshop .PSD file: you simply have to create a white layer on top of the other layers.
This removes at least the file preview, which means that you won’t the preview of your file the Finder or Explorer anymore. But… do you really need it? In my trials, this tip can save up to 30% of the file size.
Now I use this so often that I created a Photoshop Action for that. You can download it below.
What it does
The action will add a new layer in your PSD, fill it with 100% white, bring it to front.
What it does not
The action will not alter your file in another way, it just adds a white layer.
Download the Photoshop Action
Caution: I advise you to make sure everything is alright before you close Photoshop, if you just ran the action (so you could undo the action if you need it). I accept no responsibility if it fails.