What is the procedure for citing a source with incomplete information? If it matters, I am specifically asking about the IEEE referencing style, but I'm happy to learn about other styles as well.

One of the more frustrating parts of referencing is that every book publisher and journal has their own standards for what information they give you. I've come across some books that don't have a city of publication, for example, and some journal articles where the specific volume is not clear.  When citing web pages this is a particular challenge, since sometimes it is not possible to find, say, when the web page itself was published.

In cases where information is simply not available, how do you cite a source correctly?  Is there a way to indicate that the information was not available?
If you are in computer science (or anything IT related or IEEE), bibtex download (from ACM or IEEE sites) will be your friends. 

Apart from that, if you still need more information, try your best (include everything you know) and it's ok.