The semicolon: hardly anyone uses it nowadays. I use semicolons a lot, although I find it hard to explain why. It's a way of showing the connectedness of two clauses that would be lost if you used a period (full stop), where a word like "therefore" or "however" seems superfluous.

I think a semicolon captures a certain subtle inflection that we use when speaking. I don't know any hard and fast rules for using semicolons; I just put them in where I hear that inflection in my "mind's ear."