FORTUNE — Dear Annie: Ever since my spouse got laid off about a year ago, I’ve been supplementing our income by taking on some consulting

work in addition to my regular job. I’ve found that I really enjoy it, and eventually I’d like to segue into it as a full-time occupation. The question is, what do I tell my boss in the meantime?

So far, I’ve managed to fit my consulting work into my own time on evenings and weekends, and I’m very careful to avoid the use of any company resources for outside projects. (My consulting business is home-based and has a separate email address, phone line, etc.) Still, it seems dishonest not to mention to my boss that I’m doing this, especially since we are friends. Do I have an obligation to tell him? — Double Agent

Dear Double: That depends. Does your employer have a formal policy requiring that you disclose any outside employment (as many universities and some companies do), or do you have an employment contract that calls for disclosure?

If so, the decision is made for you: By keeping mum about your part-time gig, you run the risk of becoming a full-time consultant sooner than you planned.

But even if not, particularly since your boss is also your friend, “there’s a trust factor involved,” says Kristin Cardinale, author of a book called The 9-to-5 Cure: Work on Your Own Terms and Reinvent Your Life.

Cardinale knows a thing or two about wearing multiple hats. A Milwaukee-based career coach who also teaches college courses, she founded and runs both a tech support company and a national seminar firm.

“As a pre-emptive move, you could tell your boss what you’re doing, just to avoid a potentially sticky situation if he hears about it some other way,” Cardinale says. “You don’t want to seem to be doing this behind his back.”

Telling him what you’re doing doesn’t mean, however, that you have to reveal why. In these situations, as in so many others, how you phrase it makes all the difference. Saying “I’ve started my own consulting practice, and I’m working on building it into a full-time business” would probably be a mistake, since “putting it that way is likely to make your boss start thinking of you as temporary and on your way out,” Cardinale notes.

Instead, mention a current consulting project without bringing up your long-term plan. “There is little to no risk involved in saying you’ve taken on an outside gig if you emphasize that it’s short-term — say, for the next couple of months — and if you stress how it’s helping you sharpen your professional skills,” says Cardinale.

“And you’re not saying anything untrue,” she adds. “After all, until you are ready to make the leap into full-time consulting, you’re still just exploring.”

By the way, this situation is less unusual than you may think. In 2010, according to the Bureau of Labor Statistics, about 3.5 million Americans, or 4.5% of the workforce, held a full-time job while also pursuing a part-time sideline. For anyone considering doing likewise, Cardinale offers three tips:

1. Be fanatical about using your own resources, not your employer’s, for outside work. You’re already doing this, but the point bears repeating. Emailing consulting clients on the company’s system, or devoting time during work hours to meeting an outside deadline, can get you sacked.

2. Clearly define your availability. “Be upfront with your outside clients about the fact that you’re working full-time and may not always be reachable during regular business hours,” Cardinale advises. Tempting as it may be to let the boundaries blur, your full-time job has to be your top priority — at least until you decide to leave it.

3. Don’t bite off more than you can chew. Build some breathing room into your schedule so you don’t burn out. “Consulting works on referrals. You want to have enough energy to give clients your best work, so they’ll recommend you to others,” Cardinale says.

Another reason not to take on too much at once is that “while you’re still trying this out, it should be a positive experience, not one that leaves you exhausted,” she adds. Don’t worry about turning away business that, realistically, you can’t handle right now: “If you set limits — so that, when you do take on a project, you can give it your very best — clients will actually respect you more.”

Good luck!

Have you added a part-time gig to your regular full-time job? What advice would you give on how to make it work? Leave a comment below.