The best time to ask for a reference is before you need it, say just after you finish a successful project or have made a recognizable contribution at your firm.
Think about it, your boss is high on you at that moment, so are the folks who benefited from your success.
Ask them to jot down a few words in an e-mail or WORD document then save it on your hard drive. If you're worried that they'll think you're looking for a job, just tell them that an experienced recruiter suggested that you keep a proactive reference file. (That's the truth because I just did, didn't I?) 
Why do this ahead of time? Because your positive traits are fresh in everyone's heads; the reference is based on pure admiration Vs. nice guy pity (which is what bosses too often sound like once they've been forced to lay someone off.)And finally, you're not in a rush, so your boss can provide it at a time that's good for him.
One other perk, you'll know exactly what the reference says. It sure beats stressing the nitty gritty at the time that your next job depends on it!
