Everything was created in such a way that whatever can be used for good can also be used for evil. Fire and water are great examples; they can either sustain or destroy life. The individual ego is no different, where we just have to utilize it for good and beware of its negative effects when taken to an extreme. As it states in Pirkei Avot, "If I am not for myself, who will be for me? But if I am for myself, what am I?" (Ethics of the Fathers 1:14). Egos are there for a reason: so that we make something of ourselves. But we must keep our egos in check. Otherwise, we make ourselves the be all and end all, and everyone around us suffers.
Subduing the ego is quite the task, but one that is absolutely achievable. A good place to start is by realizing Who is above us: God. It's difficult to fully comprehend God because He is not human in any way. Yet, that shouldn't be a problem because we are inclined to simply emulate His characteristics. Just as God is kind, merciful and compassionate, so should we display these traits. If we're busy trying to build our own character in comparison to God, we'll become focused on goodness instead of self-aggrandizing. There's a reason why Moses was considered the humblest person who ever lived (Numbers 12:3). Since he was in constant contact with God, Moses understood how meek humans are in comparison.
Another step can be taken by considering the ramifications of our actions. As we make our way through life, we're not just representing ourselves. For better or for worse, we are a reflection of our families, friends and the Jewish people as a whole. Keeping this in mind will help guide our future behavior. Whenever the opportunity arises to do a specific action, it's important to quickly consider all the factors that will be affected besides our personal reputation. We don't live in a vacuum; our actions can adversely affect someone else.
Overall, thinking of ourselves as the center of the universe is easy to quell by simply following current events. The economy is a wreck, there are natural disasters all around us, and the Middle East is in turmoil. We are each an important piece in the puzzle of life, but simply a piece - not the whole puzzle. There is something going on that is much bigger than any one of us individually. While there are some people who think that they can take advantage of others during these times of crisis, they are actually digging themselves into a hole. When all is said and done, "the humble shall inherit the earth" (Psalms 37:11) - not those with big egos.
Ultimately, the answer of how to keep an ego in check is simple, but not easy: follow the Golden Rule. Consider how you would feel if you were on the receiving end of your own behavior. This has the power of improving one's character as well as making a person more considerate of those around them. It's the single most practical piece of advice on how to solve this problem.