Experts say the extra money is not the only factor you should consider.
"You cannot make two things your No. 1 priority," said business strategist Carol Roth, author of "The Entrepreneur Equation". "The day job comes first. The last thing you want is to fall out of favor or get demoted because you can't stay late or work on an extra project."
And experts say you should avoid choosing a second job that is too similar to your primary profession.
"You want to be sure not to get a job with a competitor," said Meade Kelley, the Chicago branch manager for staffing firm Robert Half International. "The employer you are currently with might be worried the other company could steal you. You don't want to raise concerns."
Roth said full disclosure with your primary employer is always best.
"You may find that your current employer offers to give you more projects or hours to bump up your income so you don't have to go somewhere else because they don't want to risk losing you," Roth said.
"It could be an opportunity to have a dialogue about other opportunities within your current job," Kelley said.
Factor in the added costs
That includes child care, gas, parking and a uniform. "People see a wage and think that's all going in their pocket, but they don't take into account the hidden expenses that go along with it," Roth said. "Make sure they pay off so you get enough incremental money into your pocket."
Take care of yourself
"The holidays are busy as it is," Kelley said. "If you have a full time job and want to take on an additional job, you need to be sure you're not overextending yourself."
Fatigue is not the only thing to consider. "You have to remember that your quality will sometimes decline with the quantity that you take on," Roth said. "You need to set parameters because it's not worth sacrificing your reputation or well being for pocket cash."
Remember the tax implications
"Many people don't realize that if you bring in more money, it just might bump you into a higher tax bracket," Roth said. Consult with your accountant first.
Cast a wide net
While retail jobs seem like a no-brainer, Kelley said there are other options to consider.
"This time of year we see a peak in requests for year-end accounting help," he said. Other needs: receptionists, office assistants and file clerks. "Near the end of the year, corporations are looking to get organized and have all their paperwork and accounting in order."
Consider your family
They need to be on board with your plans. "More time at work means less time for personal life, family and overall socializing," Roth said. "You may be doing this so you can get them an extra gift, but they may say, 'Forget the gift, I'd rather spend more time with you.' "