I always like to keep a high resolution copy of my documents (usually in a .psd format) and then a lower quality file to share via email or on a website (.jpg format)
Here is a really slick way to edit the resolution of a file folder full of images all at the same time.
Go to File - Scripts and select "Image Processor".
Select the folder containing your images. Then select a destination folder for your processed images.
You'll have the option of specifying the quality, image size, location of finished files and more.
What a handy feature!