The proper way to give tzedaka is described in a mishna in avos. “Hakol l’fi rov hamaaseh” which the Ra”v explains to mean: that its better to give small sums of $ to many people than to give one large sum to one person. Reason being that each time you give its another mitzvah and another time u trained urself to give. So you should gauge how much you can give a month with the amount of requests u receive and split it up. You still should use some common sense as far as importance, there are tzedakas tht are higher priority. Medical is higher than tuition for example. Also depending on the amount of money that is needed to fill the void also helps u decide how much to give.
Should you ever decline to give? If its phony and you somehow know that. Otherwise, never.