It can be very tempting to accept any job offer during these tough economic times. The difficulty people have finding any job makes it tough not to say "yes" when you are offered a position. Yet there are legitimate reasons to say reject the offer. One of these is how settling for a lesser position just to get work will appear the next time you are back on the job search.
If the job you are being offered is way below your skill level or a step down from work you had previously done, you should strongly consider turning it down. I will readily admit that it's hard to look to the future when you are trying to focus on the present, but you need to consider how taking what you can get will look on your resume. If you can only explain your decision by saying "I was desperate for a job," you need to consider other options.
You also need to think about how happy you will be at this job. It's not good for your psyche to constantly be miserable at work, and it will also increase the likelihood that you will be laid off before too long. Then you'll be back at square one and will have to explain the short employment during your next interview.
Of course, there are circumstances that would make sense for you to accept employment that is below your skill level. If the job in question focuses on a new skill you want to learn, that would be easy to explain in the future, and employers will be impressed that you were keeping your long-term goals in mind.
There are also economic circumstances to keep in mind. If you are so hard-pressed for money that you are having trouble making ends meet, then you will probably have no choice but to accept a job offer, no matter how below your expertise it may be. Yet unless you are in this kind of dire circumstance, you should strongly consider looking elsewhere if you have no satisfactory way to explain why you settled for the first job you were offered.