FORTUNE — Oh January, the month when the whole year lies ahead and — even in this dicey economy — optimism is at a peak. “Everything you want to accomplish seems doable at the beginning of January,” says Tom Connellan, author of The 1% Solution: How to Make Your Next 30 Days the Best Ever.

Great, but then reality sets in. In his consulting practice with clients like FedEx, Dell, Sony, Home Depot, and Target, Connellan has often seen progress stalled by these three mistakes:

1. Relying on “motivation” to get started

“It’s easy to forget that, while motivation leads to accomplishment, the reverse is equally true,” says Connellan. “Achieving even a small step toward a goal motivates people to keep going.”

He points to recent research showing that what motivates employees most is tangible progress. “To create that sense of forward motion, ask everyone under you to make a commitment to improve one small aspect of their job performance — whether it’s making two extra sales calls a week or speeding up a production process by 5% — within the next 60 to 90 days,” he suggests. Approaching big changes in bite-size increments “creates enough momentum,” he says, to overcome organizational inertia.

2. Thinking only in big, bold terms

Connellan says that companies’ efforts to create change go astray when “they think only about the Big Goal, not about the stages they will need to pass through along the way. But small incremental changes add up.”

Suppose, for instance, that a salesperson improves his or her closing skills by just 1% on each sales call. “Someone who makes three calls a day, and gets 1% better at each one, will double his or her closing rate in six weeks,” Connellan says.

Or let’s say that, in your last performance evaluation, you heard that it would help your team if you could listen better. “The trouble with a goal like that is that it is so nebulous, you won’t know when you get there,” Connellan says.

His solution: Start with one of the many books, tapes, or online courses available on how to be a better listener, and “make a list of two, three, or five specific things — not 10 or 20 — that you’re going to do differently.” Then concentrate on those small efforts.

3. Being impatient

This tendency is a major reason why, one British study showed, a whopping 88% of individuals’ New Year’s resolutions fail. “Even positive change feels uncomfortable for a while in the beginning,” Connellan notes. “But each time you do something differently, the brain goes to work making new connections and setting up a new habit, which will eventually feel as natural as the old one it’s replacing.”

The important word here is “eventually.” “It takes at least 21 days of conscious effort to form a new habit, but 30 days is probably a better way to look at it, because it fits more easily into your calendar,” Connellan says. “So expect there will be some discomfort with change, but commit to specific milestones for each 30-day period.”

This slow-and-steady approach can add up to measurable progress, as it did for a brokerage-firm client of Connellan’s that coached financial advisors to make “very small changes in behavior,” including emailing clients more often with information that might interest them and scheduling a few more face-to-face meetings than before. The result: sales were up 4% after six months.

“It’s fine to ‘think big’, of course,” notes Connellan. “But if you want to see huge results, start small.” That sounds like a resolution anyone, or any company, has a shot at keeping.