I use the "Save for Web" command for greater control and more precise optimization. It's found in the File menu just below the "Save As…" item.
The "Save for Web" window offers a variety of choices. Again, the options here are all fairly self explanatory.
Click on the tab at the top of the image area to choose a display option: I recommend 2-Up which allows you to see both versions of the image side by side.
The area below each image in the "Save for Web" dialog box gives you information concerning how the file will perform online. Some items you should see include the original image filename and file size; and the optimized image optimization options, the size of the optimized file, and the estimated download time using a selected modem speed. I use the 28.8 setting to be sure.
Now play with the "quality" options to the right and test the different settings.
Nothing will actually change or affect the file until you select "Save Optimized" — so play around until you have a good visual understanding of what to expect.
You can also set your scaling at this time and Photoshop does a good job of sampling your image down to the correct size. Remember: never enlarge.