I've heard some names pronounced with 〜さん added to them (such as 佐々木さん) and some without it. I believe it is related to respect or the age of the person named. What would be the guidelines or general principles to follow regarding 〜さん?
"-さん" is an honorific suffix added to give respect. It can be used either with males and females, and also with given names and family names, not to your own name, though.
It can be even used attached to the name of the occupation and titles.
It's ok to use it with people that you are familiar with, but it's kind of mandatory when you are talking to people you aren't familiar with or people that are not close.
Finally, as stated in this page about the "Proper use of "-san"", don't drop the -さん suffix unless you're being specifically invited to. You can see further info in that link I just gave you.
If you're talking to somebody who is not an extremely good friend, use さん, but if you're about to start a fight with someone, you should drop the さん. If you're a foreigner and are on good terms with someone, you can often use their first name without さん, but using their family name like that will sound rude and abrasive. If you need to attract someone's attention in an emergency situation, you can drop the さん too, as well as adding an imperitive (おい、山田！速く逃げろ！).
One of the exceptions is when talking about someone in your group (company or friends) to someone outside your group. In that case, even when talking about your boss, you drop the さん (or 社長, 部長, etc.) and just refer to them by their name. So if I'm talking to 鈴木さん (a person at another company) about my boss, Yamada, I would say "山田", not "山田さん". (I'm not certain about this rule, so correct me if I'm wrong).
If you're talking with someone you don't really know, you definitely want to add さん. If you add it for friends and family, it might upset them as it may make the two of you seem a little distant with one another.
As to your age/respect thing, if you're below them, then just add さん. Like Matti said above, if you're just starting, you can't really go wrong with appending the さん after the name.