Rav Haim of Volozhin (1749-1821), in his famous work Nefesh Ha’haim (3:3), mentions that contemplating the words "En Od Milebado" – that there is no power or force in the world besides G-d – is an effective Segula against harm. This concept already appears in the Gemara, which tells that Rav Hanina Ben Dosa was unharmed when sorcerers tried to cast a spell on him, because he said, "En Od Milebado," reaffirming his belief that only G-d controls the universe, and there is no force capable of overpowering Him. Rav Haim writes that contemplating these words and their meaning is an effective Segula to ensure that nobody exerts any sort of control or power over a person (listen to audio recording for precise citation).
Accordingly, it has become common for people to keep near them cards or stickers with the words "En Od Milebado," so that whenever they find themselves in any sort of difficult situation they will be reminded of this concept and thus make use of this special Segula. People keep these in their wallets, on their refrigerators, in their cars, and other places so they will frequently be reminded of this concept.
It should be noted, however, that these cards and stickers require Geniza (burial), since these three words come from a Pasuk in the Torah. Once they start to fade and one wants to replace them, they may not be thrown it in the trash; they must be placed together with other sacred texts which will be collected for burial. This is the ruling of Hacham Moshe Shayo in his Mehkereh Eretz (vol. 4, Y.D. 30).
Summary: Many people have cards or stickers with the words "En Od Milebado" because contemplating these words is an effective Segula for protection against harm. One must ensure not to throw out these stickers or cards, and to rather put them in a Geniza.
