In both my work as an editor (for the great little company, Darling Axe), and as a former English teacher at Yukon College (now University), I encountered a lot of writers who weren't quite sure when to use the semicolon. People tend to think they can insert it where a comma goes, but the semicolon has a bit more punch than a comma. As a student of mine once put it, "It's not a red light, like the period; it's not a yellow, like the comma; it's more like the Quebecois 'rolling stop'." (Note what I did there with all the semi-colons.) And he was totally right and kind of funny, and hopefully not offensive to those living and driving in Quebec.
The semi-colon should be placed where a sentence might go, but it connects two complete thoughts that are made stronger by being joined together. You can't have an incomplete sentence on either side of that semicolon.
It can be useful to think of writing in terms of sound and punctuation like markers of rhythm. The semicolon is signalling to you to read the two sentences it joins together with a quicker beat between them. Don't take a full breath as you would with a period. Read on, knowing you have two perfectly whole thoughts that lean on each other to become something greater than the sum of their parts. It's kind of like two sentences decide to get married and the semicolon is where they join hands. All the magic happens in that union.
