This week's Guardian Guide has a fascinating article on TV theme tunes by Daniel Pemberton, himself a composer of the genre.
It contains eight tips on composing theme tunes, and narrates some good and bad examples in each category.
One tip is "Do: write quickly". Apparently the "Grange Hill" theme tune was composed in a few minutes. The composer Alan Hawkshaw didn't know what he was composing it for. It ended up as the "Grange Hill" theme tune and the "Give us a Clue" people later said they wanted it.
Another tip is "Do: try to get the theme tune played as often as possible". He refers to the Big Brother theme tune by Paul Oakenfold. Pemberton found it "pish" to start with, but it has been repeated so often that it is now ingrained in the national consciousness and regarded as a work of genius.