Often I have a significant amount of time invested in my PSD files, and want all layers, effects, filters ect. preserved in the re-sized file. I'm sure this seems obvious but considering your question; A word of warning... before making any changes... do a "Saved As" and re-name your file BEFORE making your changes. Otherwise you risk, saving over your original file. It's easy to forget and mistakenly hit "save" once you start resizing or modifying the file.
If you're a MAC user, time machine can be a life saver, but even if you do auto back-up every few hours... you're still a risk of losing all your work between back-ups.