Can I give ma'aser on money that I didn't earn yet but I know I will be earning in the future?
According to the Chofetz Chaim (Ahavas Chessed 18:2), this is subject to a dispute among the poskim. In order to avoid this problem, the Chofetz Chaim recommends that when one starts to distribute maaser, he should explicitly say that he is accepting this practice on the condition that he can distribute tzedakah before earning the money.
The Tzedakah U'Mishpat (5:11) explains the above applies only if one gave the money to tzedakah without any preconditions and only later decided to have it be considered maaser. However, if he explicitly gave the tzedakah with the intention of it being considered maaser on future earnings, one may deduct it from his future obligation.
When I started keeping track of ma'aser, I set up a dummy account in Quicken to maintain the records. Since it is a dummy "account" it can go negative. As I earn money, I add the appropriate ma'aaser to the record. In effect, this is just a way of ensuring that I always do give sufficient tzedakah. My Rav had also spoken of actually setting up a special account to contain all the money to be given to tzedakah but this way is easier so that the ma'aser ends up correct. In any case, money is taken and put into the ma'aser account as it comes in so that it is easier to keep track of what is owed. Part of the tnai is that at the end of the year, I zero out the account by saying that all money over the correct ma'aser is tzedakah.
In any case, what people who have done this have found is that they wind up being able to support themselves on what is left more easily. This method actually brings a bracha on the person.