Despite what you might have heard, many benefit packages aren't flexible, says Ms. de Lande Long. So, while it's worth asking, it might be difficult to modify the health plan. Your success in getting more vacation days depends on the employer, says Ms. de Lande Long.

Your potential boss might be hesitant to give you more days if it will make other employees think they're being treated unfairly. Instead, focus on things that are easy for the employer to provide, such as a work-from-home arrangement for one day a week, if the employer has made such arrangements in the past, says Mr. Akana.

If you still feel your package is too low, ask if it can be reviewed again in six months. "That way, you can show them that you're worth the money," he says.