If you’re like most professionals, you’re probably very skilled at setting goals–but not quite so adept at achieving them.

With the best of intentions, we make New Year’s resolutions, vowing to lose weight, exercise more, or get organized. And by February, our resolve usually evaporates, along with our goals, according to a poll by the time management firm Franklin Covey. The company polled more than 15,000 customers about their New Year’s resolutions and found 4 out of 5 eventually break them. About one in three didn’t even make it to the end of January.

At work, you’ve probably seen the same phenomenon. Companies establish all kinds of goals–from lofty missions statements to specific growth targets–then often fail to meet most of them.

Why? And what stands in the way of achieving goals? Here are five common mistakes:

1. You underestimate how hard it is to achieve the goal.

Those flabby abs? They can’t be turned into a sexy six pack in six days. Those diet books that promise full body make overs in 30 days? Not going to happen.

In reality, most meaningful goals take a lot of work to realize. If you don’t recognize from the out start that losing ten pounds, or increasing sales 10%, will take considerable time and effort, you will find it all too easy to give up once you get caught up in the day-to-day. You have to forecast the difficulties so that you are mentally prepared to meet the challenges when they inevitably arise.

2. You didn’t “own” your goal.

“I’m just doing this because my boss wants me to” is a goal that is destined for failure. If you’re just implementing a new sales strategy to please the new vice president, not because you believe in its necessity, you’re going to find it impossible to stay on course when you encounter obstacles–or just the daily interruptions.

We’re living in a perfect storm of distractions–email, cell phones, texting, IM, on demand media. It’s way too tempting to tell yourself, “I’m incredibly busy, I’ll get to this tomorrow.” In one survey, people admitted to wasting nearly two hours a day of an 8-hour work day on socializing or goofing off on the internet. Waiting for a “tomorrow” usually means never.

If you want to meet that 10% target, you need to be self-motivated and be committed to achieving it.

3. Your goal wasn’t clear, or measurable.

“Increasing customer satisfaction” is too general. You need to identify the specific, quantifiable goal (ie: improving customer retention by 5 percent), so that you can measure your progress on a regular basis. The on-going monitoring–seeing that retention inched up, or down–will reinforce your strategy and help you stay on track.

Yes, we know that there are people who argue that dieters should never get on a scale–that you can tell if you’re losing weight by how your clothes fit. But how many people actually lose weight that way? And is it really possible to keep focused on that difficult-to-achieve goal, without periodically checking in to see how you’re progressing?

4. You didn’t realize the rewards would be modest.

If you set a goal to increase sales 10%, and so far you’ve inched up sales 2%, you’re probably not going to see the confetti sprinkling down over your head. The sense of satisfaction may be limited. Progress frequently is incremental, and slower than we hope. The key is to remember that fact, so you keep plugging on.

5. You tried to do it alone.

There is a very good reason why so many diet plans encourage dieters to join to support groups. Most of us need a community of supporters who will cheer us on when the going gets tough–and, most importantly, hold us accountable. Just the sheer act of publicly acknowledging your goal can help make you accountable to achieve it.

It takes courage–and humility–to publicly admit that you need to do better. But once you do, having that band of supporters will help you stay disciplined to reach your goal.