1. Don't gloat. Try and at least appear as if you are not enjoying it.
2. Always remember that the guy across the desk who you are stitching up today might eventually end up being your own boss one day in the future. Bad karma usually means bad news.
3. Never start off the conversation by seeking to gain sympathy from your team member by claiming that your own bonus was bad. Lying is never cool, and most of your subordinates will probably feel that you are worth no bonus at all in any case.
4. Always conduct the meeting alone (without HR). That way, it will be your word against that of your subordinate - especially if things become strained, and ends up with violence. And who needs HR obtaining yet more evidence that you are a poor communicator and a lousy manager ?
5. Never try to rationalize your decision. Trying to be too transparent about how bonuses are calculated will only serve to highlight the fact that the process is 100% unscientific, and depends mostly on who you know, and how much you brown-nosed them throughout the year.
6. Never promise your disappointed subordinate that you will make it up to him / her next year. He or she will remember only too well that this is the exact same thing you promised them you'd do last year.