Weaknesses
Like any respectable villain, Mara has at least one essential weakness - an Achilles' Heal, if you will. Make that several. Mara's vulnerabilities are exploited repeatedly for comic effect throughout the manga. Here is a list of Mara's most important weaknesses.
Good-Luck Charms.
Mara is severely allergic to good-luck charms. In chapter 29, when she accidentally steps on a hamaya (lucky arrow) and then bumps into a lucky bamboo pole, Mara receives an electric shock so great that it causes her to pass out. When she awakens, she has not only lost all memory of who she is and what she's supposed to be doing, but she has also turned into an incredibly nice, helpful (albeit befuddled) person. Belldandy says that Mara's new attitude must be an effect of the hamaya she stepped on. Later, Belldandy throws a lucky bear statue at Mara, and when Mara catches it, she is shocked back into regaining her memory.
Mara's aversion to lucky charms is exploited several times later in the series. For example, in chapter 45, when Mara possesses the body of Megumi Morisato, Belldandy forces Mara out of Megumi's body by offering her a cup of tea with a lucky tea stalk floating in it. The thought of drinking lucky tea makes Mara violently sick, and she emerges from Megumi's body, coughing and gagging. Much later, in chapter 159, Skuld and Keiichi (and Banpei and Sigel) completely incapacitate Mara by tying her up and pressing good-luck charms against her body, which causes her to become paralyzed and confused.
Rock Music.
When Mara hears rock music, she dances uncontrollably, and can't stop herself until the music stops. In the English translation of the manga, "rock music" is changed to "disco music," which is even funnier considering the way that Mara dances. It is loosely implied that all demons share some sort of musical weakness - for example, Urd, who is half-demon, falls asleep whenever she hears Japanese enka ("polka music" in the English translation). Unlike Urd, none of the other Goddesses exhibit any similar music-related weaknesses. So maybe a musical weakness is particular to those of demon blood. Who knows?
The Gods CD.
The Demons CD, the catalyst that summons Mara to Earth, is actually part of a 2-CD set that includes the Gods CD. The Gods CD, when wielded by a Goddess like Belldandy, is powerful enough to seal up Mara for 500 years. In addition, the Gods and Demons CD form an inseparable pair, which means that unless Mara has both under her control, she can never truly be free of the threat of the Gods CD being used against her. (Mara can use the power of the Demons CD to combat the Gods CD, but she's smart enough to realize that it would be better to just have both.) Unfortunately, at the end of chapter 28, a monster (that Urd cooked up in the kitchen, no less) suddenly appears and snatches the Demons CD right out of Mara's hand, swallowing it. Neither the Gods CD nor the Demons CD is ever mentioned again in the manga. Even though Belldandy still possesses the Gods CD, it is implied that she can never use it against Mara again, because "the Demons CD and the Gods CD only take effect... when they're together," and the Demons CD is now presumed to be gone forever.
Video Games.
This isn't a real weakness, per se, but... Mara isn't very good at video games, even though she fancies herself to be. In chapter 60, Urd effortlessly beats her at a game of Tetris (Mara can't even get past level one). And in chapter 139, Hild kicks her ass in a combat game. "You have no skill at games," Hild tells Mara, "So it'd probably be best for you to just stop." Despite the fact that she's not good at video games, Mara does spend an awful lot of time playing them. She is determined to someday beat Urd at Tetris.