It depends what software you are using. If you are using Photoshop, then use the "Save for Web" option by going File >> Save for Web or Alt+Shift+Ctrl+S.
The benefit of this is that you can then control pretty much all aspects of the picture, for example pixel size (via the Image Size tab), file type (jpg, gif, png etc) and quality. You can also save your settings if you have a setting you use regularly.
It's also a great tool to use if you are posting images on POTN to ensure you are under 800px/100kb limit; as you adjust the Quality slider it will tell you the file size (K) in the bottom left corner.
N.B. After resizing you will see some slight degredation in IQ, so it can help to sharpen or USM after resizing as long as you aren't concerned about file size (sharpening will increase file size).
Hope this helps!