The Rebbe's guidance on how much to give and in what spirit:
Generously — even beyond your means. Halachah sets minimums, but Jewish compassion goes beyond them (IK 271). Giving generously, even beyond what seems possible, with trust in G-d, opens new channels of blessing (IK 720, 4970, 8222). Tzedakah is the one mitzvah in which we may "test" G-d's blessing (IK 5809). Giving even when it is hard for you has special merit (IK 1116, 1371). At the same time, there are practical limits to giving (IK 1572; see IK 4993 on the halachic limit of a fifth).
Maaser. Give at least a tenth — and a little more than exactly a tenth (IK 2868). Start giving maaser from your profits, even with a small symbolic amount (IK 10254). Keep giving maaser even in financial uncertainty or hardship (IK 6248, 6873). Take maaser from a new business (IK 5915), from payments on a home or a building (IK 2308, 3185), and from compensation or reparations received (IK 6581, 7177). The Sages' promise "give a tenth so that you will become wealthy" is meant literally (IK 2404, 3186). See also: Livelihood – Giving charity and maaser.
Promptly, and keep your word. Act quickly in tzedakah; promptness brings great merit and more opportunities to give (IK 995, 2147, 10333). Fulfill pledges promptly and with enthusiasm — delaying a pledge delays the blessing (IK 2827, 5655). Keep special donations separate from regular giving (IK 6883). Don't give with vows (IK 6565), and don't make conditional "deals" with G-d (IK 1353). When new requests come, don't cut previous commitments — try to widen your capacity instead (IK 8548); in hard times, maintain or even increase your giving (IK 2194).
With joy, simply and discreetly. Give with joy (IK 6513, 9402), wholeheartedly and with self-sacrifice (IK 8625), without expecting anything in return (IK 10740). Hidden tzedakah has immense merit (IK 1100, 3239). Don't over-analyze: support all proper causes with a set minimum and add more where you can (IK 7175); as on Purim, give to whoever asks (IK 7583). Giving during one's lifetime brings more blessing than postponing it (IK 5147). Even tzedakah given for personal motives is precious (IK 6791), and a sincere wish to give is itself valued by G-d (IK 1569, 4951). There must be no hint of pressure in collecting tzedakah (IK 8065).
See also: Tzedakah – Giving every day and at special times; Tzedakah – Where to give.