I've noticed my level varies greatly from day to day, depending on my diet and how much exercise I've gotten. I decided to do some research to see how I could improve it.
I should note I am not a physician and am not qualified to offer medical advice. This is merely a summary of what I've been reading from various sources. If you have any questions or concerns, please be sure to see your doctor.
Women tend to be deficient more often than men, due to the loss during their monthly cycles.
Vegetarians and vegans also have to pay special attention to the iron content in their diets. While plentiful in many plant-based foods, it isn't absorbed as readily by the body as the iron in meat.
An easy trick: Eat foods rich in vitamin C at the same time as the iron source. For example, if you are eating tofu or spinach, both solid iron sources, squeeze some fresh lemon juice in your water, add strips of green pepper to your main entree or eat strawberries for dessert. The vitamin C will help you absorb the iron in the other foods better. Also, for spinach, it's a good idea to cook it. This alters the iron content to make it more available to the body. And try to avoid caffeine shortly before, during or after the meal, as it is known to inhibit absorption.
Another interesting thing I've read is that strenuous exercise can contribute to iron loss, and even deficiencies, especially in women. There are several reasons. For one thing, iron is a major player during processes involved in energy use and oxygen distribution to tissues, so this increases an athlete's need and use as his or her metabolic rate rises. It also appears some iron is lost through sweat and the digestive tract. One especially fascinating phenomenon in runners is that the impact of their foot strikes can cause red blood cells to burst. The iron the cells contain is then apparently lost.
You can read more about iron deficiency and exercise here and here. The second article is especially informative, but please note it is on PowerBar's website, which has a stake in the matter. Regardless, the content reflects information I have seen on other websites, and it strikes me as being sound.
If you are afraid you might be deficient, a word of caution: Don't take any iron supplements without seeing a doctor first. Iron excesses can be extremely dangerous.