In addition to a minimum requirement [which we discussed last week], Chazal instituted a maximum amount that may be given; one should not give more than 20% of his money. The reason is, in order that the donor himself will not become poor and need to rely on other people (Gemara Kesubos 50a & Rema YD 249:1). However, there are exceptions when he may give more. Many of these circumstances are subject to dispute and therefore a Rav should be consulted. Exceptions include: one who is extremely wealthy, before one dies, as an atonement for one's sins , if the poor person is in front of you [and you can afford it], pikuach nefesh (saving one's life), pidyon shivuyim (redeeming captives),supporting Torah institutions and Torah scholars, one who has a steady income, money not earned [e.g. found or inherited] and someone who in general wastes his money (see Laws of Tzedakah and Maaser pg. 80-85 for sources).
Although the implication of the Shulchan Aruch [YD 249:1] that "One who gives 20% fulfills the mitzvah to the highest level" is that giving 20% is optional and not mandatory, there is a dispute in a case where the poor person is in front of you [and is not capable of supporting himself (e.g. young orphans)], whether there is an obligation to give 20% or it is praiseworthy (Ahavas Chesed 2:19:4). Additionally, there are many opinions that hold as long as one knows of a poor person, it is considered as if he is physically in front of you. Therefore, if he receives a letter [signed by reputable Rabbanim] in the mail, informing about someone's needs, one would be obligated to contribute [assuming he can afford it] (Shu"T Be'er Moshe 4:92). Even if one is not able to contribute, it is still befitting to daven that this poor person be relieved from their financial plight (Priorities in Tzedakah pg. 45).
It is important to keep in mind, when faced with a situation of giving tzedakah, even if one does not have what to give, he should appease the poor person with words (Rambam Hil' Matnas Aniyim 7:7). If however, he discovers that a person collecting tzedakah is a fraud, it would be permitted to chastise him (Aruch Hashulchan YD 249:3).
The Rambam [Hil' Matnas Aniyim 10:7-15] and Shulchan Aruch [YD 249:6-13] list eight levels of how one can fulfill the mitzvah of tzedakah [the order is written from highest to lowest, for the more one can avoid embarrassing the recipient the better, as we try to imitate Hashem as much possible]: 1) Make it possible for a person not to need tzedakah. Included is anything that would help someone be self-sufficient, both before or after one becomes poor (e.g., find them a job, giving someone a gift or loan and allowing him to pay back over time, and supporting Jewish businesses), because by fulfilling tzedakah this way, it allows the recipient to maintain his dignity, 2) both the donor and the recipient do not know to whom or from whom the money is going, 3) the donor knows who is receiving the money, but the recipient does not know whom the donor is, 4) the recipient knows who the donor is, but the donor does not know the recipient is, 5) giving money to a poor person, before the poor person asks for help [this is less embarrassing for the recipient, since he did not have to ask for the money], 6) after the poor person asks for help, give him all of what he needs, 7) after the poor person asks for help, give him some of what he needs with a happy face, and 8) after the poor person asks for help, give him with a heavy heart [but the poor person does not realize his reluctance].
It is important to note, if one gives tzedakah and shows his displeasure, he loses his reward for the mitzvah (Shulchan Aruch YD 249:3). This is in addition to violating a negative commandment (Shach YD 249:5). Additionally, the Zohar writes there is a level in Gehinnom for people who showed brazenness to poor people (Derech Emunah 10:ftnt. 16).