Besides Cropping and Undo'ing, I have found the following steps are less error-prone:
- Adjust the layers you need
- Make a marquee selection
- Ensure a visible layer is being selected, use Edit -> Copy Merged (Shortcut, Cmd-SHIFT-C, or CTRL-Shift-C)
- Create a new document (Cmd-N or Ctrl-N), Photoshop will automatically set the document to the size of the selection
- Paste the image
- Perform a Save for Web (Cmd-Option-Shift-S or CTRL-ALT-SHIFT-S)
I use the above steps a lot over the past number of years as it involves shortcut keys, hence they speed the workflow up and offer the convenience of speed and precision. I know creating a new doc may not be what you wanted, but the approach may actually end up saving you some time.