If you're using a 'destructive' workflow, one in which each step changes the pixels--Photoshop's default way of working--order of operations can have a strong impact on the result:
1) Getting it right at the source is key--do a high-bit depth scan, use digital ICE if available.
1b) Capture in the lowest gamma, broadest color space you can (ie. AdobeRGB or better yet ProPhoto RGB), convert to it now if you can't.
2) Some applications (or plugins for Photoshop) allow you to do something called "capture sharpening", often implemented as Richardson-Lucy deconvolution (sounds scarier than it is to use). Done correctly, deconvolution is actually restoring high frequency information (detail) to your file (it's providing real information, not the illusion of detail as unsharp mask does). You may find that a capture sharpen of your scans can help you get a sharper result, without any halos, crunchiness, or eye fatigue from the resulting work.
3) Do a proper (correlated color temperature) white balance of your image (if no true white balance tool is available, then color balance as part of step 4). You'll have a dedicated tool labeled white balance, grey balance or neutral picker, if you have one. Interestingly, Photoshop does not have a white balance tool, but Adobe Camera Raw does. You can get your scan into ACR with some contortions, if you wish to.
4) Use curves to do your color balance, gamma correction, level correction, exposure, contrast, and tone mapping. Order of these won't matter as all will become a single curve operation.
5) Apply your creative effects--saturation, sharpening, vignetting, et. al. The fewer "commits" the better, for image quality.
6) Optimize your content for output; web version, print version, digital display versions of the same file will all be prepped differently. Don't forget to change to an appropriate color space for presentation. If you are interpolating (upsizing or downsizing), use bicubic or better (bicubic is Photoshop's best so that's a great starting place).
Sharpening is a mystery to most of us, but when done correctly is invisible. Worth a read: http://www.amazon.com/World-Sharpening-Photoshop-Camera-Lightroom/dp/0321637550 (no affiliation).