When a mentor asked Microsoft software engineer Steve Fernholz how much money it would take for him to quit his day job, he had a ready answer: $100,000. She wrote him a check on the spot for $25,000, which was enough. He left his job to work full time on Droplet, a yard sprinkler system. The first products shipped in July 2014, and by November the company had raised $450,000.

The signs that it's time to abandon your day job aren't always quite that obvious. Most entrepreneurs see smaller hints. Pay attention to the subtle clues--they're not just telling you it's time to take the leap, but also pointing you toward the preparations you must make before you do.

The Sign: You're starting to do the math

You've run the numbers on how much to put away, and how much revenue your company must generate. But don't guess--create a business plan instead. Sabrina Parsons, CEO of online business plan maker Palo Alto Software, says it should include an analysis of the problem you're solving, your solution, your target market, a competitive assessment, a marketing plan, and a financial forecast with cash-flow projections. Such a plan likely saved energy bar maker Picky Bars from running out of cash, says CEO and co-founder Jesse Thomas. He has an MBA, but didn't realize his side business had a 60- to 90-day lag between buying supplies and ringing up sales. Absent changes, "we wouldn't have had enough money for ingredients," says Thomas, who runs Picky Bars with his wife and a friend. So, they switched to a subscription model. For help with a plan, tap online resources (Palo Alto Software's LivePlan offers 500 sample plans with numbers from real businesses) or contact a Small Business Development Center or Service Corps of Retired Executives office.

The Sign: You're working two full-time jobs

Though it's hard to do, be sure to plan as much as possible while you're still on someone else's payroll. Fernholz built prototypes of his sprinkler and read a ton about irrigation while at Microsoft.

When you've grown your business as much as you can in your off-hours, it's time to jump. Julie A. Lichty made the switch from corporate executive to full-time business coach only after she realized her new firm, based in York, Pennsylvania, needed more of her time to grow. Beth Briggs organized every aspect of her now-shuttered AquaDog Spa--including financing, construction, and computer systems--before quitting her day job.

The Sign: Your side business is starting to get love

Outside validation can give you the push you need to quit a day job, but it's also a powerful business tool. Andrew and Mary Ross made more money at their business, Sensory Swim, which offers swimming lessons for children with autism, than from their salaries. But it wasn't until the owner of a swimming pool franchise gave the couple access to any of his pools--citing the company's great reputation--that Mary left her job. "I just knew we had to take the leap," she says.

Now turn that love into money. Keep track of satisfied clients, favorable terms from a supplier, Facebook likes, and any media mentions, ask to use positive comments, and then spin them into advertising. Publish favorable quotes or articles on your webpage, tell suppliers about the great terms you've been extended by other vendors, and shamelessly self-promote. It will make your company look bigger than it is (for now), and it's the best kind of PR--free, which may be the only kind you can afford for your new venture

From the February 2015 issue of Inc. magazine