The best advice I can give on that is to practice and play with all the different adjustments you can make. Levels, curves, and the other colour adjustments are a good place to start. I now do almost all of my post work in Apple Aperture. Adobe Lightroom does the same things too. Way easier and faster than using Photoshop plus they help organize and sort your photos making the selection process easier. All the adjustments are in the same place too instead of spread throughout different menus.
You will also want to play with the retouch brush (spot healing tool in photoshop). It's great for removing blemishes. If your portraits are more modelling type shots, you could look at Portrait Professional Studio. It handles making people's complexion look perfect.
Absolutely. Any post processing is good in moderation. Too much can actually make the photo worse. Especially if you shoot in JPEG instead of RAW. Keep things more subtle and they will look better.
It's also quite helpful to have a reference such as Kodak or a colour corrected previous image to work side by side with - i find it keeps me reigned in with the processing lol