Rabbi Elazar of Bartota said:
Give to Him of that which is His,
for you and whatever is yours are His....
Pirkei Avot 3:7
Do not begrudge giving to charity, since you are not giving away your possessions but rather those of G-d (Meiri). And give humbly, since the fact that you own money to give away--and that you have the free choice to do so (Avodat Yisrael)--are powers granted to you by G-d (The Rebbe).
The Mishnah does not only speak of giving money, it speaks of giving of yourself, as well. Realize that your entire existence depends upon G-d--that you and your entire reality would be nonexistent without Him. You will then be more willing to sacrifice your own comforts and desires for the sake of G-dly ideals. Recognize that all you posses--wealth, strength, wisdom, artistic talents, etc.--are a gift from G-d and that it is only proper that you use them to serve G-d. (Midrash Shmuel, Tiferet Yisrael).
Rabbi Yosef Yitzchak of Lubavitch used to say: When you make a decision to give to charity and to help another person generously--even if your decision does not reflect your available means--new channels are created on High to produce all that is needed for the fulfillment of your good resolution. (Igrot Kodesh)
Holtzberg Memorial Edition