If there's one thing Americans on a whole are bad at, it's taking time away from the office. Last year, 21% of U.S. workers left vacation days on the table, and the reasons ran the gamut from having too much to do at the office to not wanting to deal with the aftermath of going away.

But what happens when you try to take vacation, only to have your requests continuously denied? The last thing you want is to not use your paid time off, but how do you take advantage of your vacation days when your manager never lets you take them? If you're stuck in this tricky situation, here are a few tactics that might help.

Professional woman at computer with angry expression

IMAGE SOURCE: GETTY IMAGES.

1. Ask for time off during slower periods at work

Requesting time off during the busiest periods of the year means putting your manager in a tough spot. Your boss might want to give you that time off, but if doing so puts the business at risk, he or she can't afford to do it. So don't set yourself up for disappointment and put your boss in a position where he or she needs to make you unhappy. Instead, plan your time away from the office during calmer periods. For example, if work tends to slow down during the summer, ask for some vacation time then. Similarly, avoid asking for time off during the winter holidays, when there's a good chance the majority of your colleagues are clamoring for it as well.

2. Request time off after completing major projects

Your boss might hesitate to grant you time off if he or she knows that you're needed for a big project that's still in the works. A better bet, therefore, is to request time off after major initiatives are completed successfully. For example, if you're an IT manager who's tasked with rolling out a new software for the entire company to use, wait until that system is up and running bug-free to ask for a few days out of the office. Not only will your manager be less nervous about saying yes, but he or she will have a harder time denying your request on the heels of a major effort and win.

3. Enlist some backup help before talking to your boss about vacation time

If you're responsible for a number of important tasks, then your boss might hesitate to let you take time off for fear that things will just collapse in your absence. That's why it pays to come to your manager with a backup system of sorts already in place before requesting time off. For example, imagine that a big part of your job is to provide your company with marketing data -- data it needs on a consistent basis. If you train a colleague to pull that data and package it up in a neat report, and then let your boss know that you've taken that step, your manager might loosen the reins and grant that much-needed escape.

Not only do you deserve time off, but you probably need it to maintain your sanity. If your boss keeps saying no every time you ask for vacation, and the above tactics don't help, then you may have no choice but to take the matter to your human resources department. This especially holds true if your company has a use-it-or-lose-it vacation day policy, and you can't carry time off from one calendar year to the next. The last thing you want is to miss out on a key benefit you're entitled to just like everybody else.

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