It's a cliché, but that's because it's true: bosses like staff who bring them solutions, not problems. If you approach your boss to ask about promotion in the middle of a major project or just before an important deadline, you'll be creating a problem and possibly causing permanent damage to your chances of advancement.
So choose your moment carefully. Take into account the organisation's work cycle, your boss's own work cycle, and your boss's temperament, and calculate the best moment to receive a positive, unhurried hearing. The best of all possible times will be just after you've completed a piece of work that has made you stand out, or when you've comfortably exceeded your targets.
But even if you've simply been doing your job, doing it properly and doing it for long enough, you're perfectly entitled to want to progress. The message you want to send is: I've served the organisation exceptionally well where I am, but I could do so much more if I were promoted.
The key thought to keep in mind through the whole process is that no one ever got anywhere by waiting to be asked. If you want to make progress, you've got to ask for it. Don't be shy.
