Henry Watson Fowler* said we should use a colon ‘to deliver the goods that have been invoiced in the preceding words’. So, use them:
To introduce a list
‘The Writer does four things: consultancy, training, naming, and writing.’
To add to what you've just said
‘It was a tough job spec: applicants had to have at least three years’ experience and an appreciation of salted caramel ice cream.’
To introduce other sentences/questions
‘Alex had to choose: The latest Marian Keyes for book club? Or something more high-brow?’
For steps (also uses semicolons—see below)
‘If you’re the last person out of the office, follow these steps: first, rejoice at having finished for the day, even if it is five hours after everyone else has left; second, close the windows; third, switch off the lights; fourth, set the alarm.’
Use a semicolon to link two separate sentences that are closely related, e.g. ‘Ella was back at work today; she’d spent the previous week by the seaside’.
Use it in a list that already contains commas, eg ‘On Harry’s desk you’ll find: The Oxford English Dictionary, for his wordy needs; the remnants of a pot plant, long-since dead; and an empty mug, crying out to be filled up with tea (milk, no sugar).’
*A guru of words, described by The Times as ‘a lexicographical genius’.