Freckles are caused by sun exposure, and people who are more susceptible to sunburn are more likely to accumulate freckles. For example, because their genetic makeup gives them pale skin, redheads often have freckles. If you're someone who tends to collect freckles, after you do away with them, you'll probably accumulate replacements unless you change something.
Vitamin C not only prevents germs from infecting you and making you sick, it also prevents the sun from adversely affecting your skin. It literally prevents pigment from clustering into freckles. Make sure to consume at least the daily-recommended U.S. Department of Agriculture amount of vitamin C, either in foods or as a supplement. Similarly, vitamin E is thought to gradually equalize pigmentation. In other words, it makes your skin more uniform in tone.
And, of course, wear sunscreen of at least SPF 15 whenever you're going to be out in the sun for even a short amount of time.