What is giving charity (or donation) and how to get the best possible result (good Karma) from a donation?

When you do a giving (as a donation), to become a powerful good Karma (to get better return in the cycle of rebirth) there are three factors. First one is Chiththa-Sampaththiya, it means giving with a clear mind and believes that there’s a resulting Karma. As much as he knows about case and effect (Karma), result is powerful in good side. Before the giving, when giving and after the giving, the mind should be clear and happy about it. There should be no hidden goal such as becoming famous by giving. If you have a goal of becoming famous by giving, then the resulting Karma is less powerful. It is said that if someone gives with the mind of "With this good Karma I may receive eternal happiness (Nirvana)" it is the best possible giving that can support you in the cycle of rebirth. "With this good Karma I may rebirth in a heaven" is not that good. The second one is Vasthu-Sampaththiya. It means giving thing should be correctly received your own thing. Also the things you are giving should be arranged in a better way, you should give in a honorable way, you should give by your own hand, you should give something that is not useless (If you give a useless thing still there is a result but not a better result). Third thing is Prathigrahaka-Sampathhiya. It means the receiving person should have a good virtue., the best possible receiver is a Lord Buddha with Sangha (Sanghika Dana). If you could give one spoon of food to a Lord Buddha, the Karma from that deed is enough to be in heavens for many eons and become human king for more than 1000 times. But giving Dharma (ultimate reality) knowledge is the most powerful giving. It is said that giving Dharma knowledge is better than giving life (protecting his life). If you do a giving without knowing these things, there is a result but not a good one. 
But when you are killing if you know about Karama the bad result is reduced, since you are afraid of killing. When you kill with a wicked and angry mind the result is powerful in a bad way. When you kill a large animal you have to put great effort. So when it is large, Karama is powerful in a bad form. Also, Karama will be powerful in a bad form when the receiver is virtuous. No one (a god, Mara or a Brahma) can kill a Lord Buddha so the result is not explained. There are many stories where people went to hells just by blaming Lord Buddha.
Giving without seeking anything in return leads to greater spiritual wealth and it reduces the greed (Lobha), But giving with the mind of "With this good Karma I may receive eternal happiness Nirvana"(without expecting any worldly things) is the best as it returns good deeds in the cycle of rebirth and also help to achieve Nirvana. To reach eternal happiness, all beings must complete Giving in some level as Pāramitā. To attain the Buddhahood, anyone must fulfill giving (donation/Dānapāramī) in grate (unthinkable) manner in three ways. Regular Perfection(Pāramī), Higher Perfection (UpaPāramī) and Highest perfection (ParamatthaPāramī).Giving out some objects(outside of the body) such as wives, children, assets are Dānapāramī; giving organs such as hands, feet, ,etc., is the DānaUpaPāramī; giving own life is DānaParamatthaPāramī.
Lord Buddha may have donated only blood more than the amount of the sea to complete the necessary Pāramī in the cycle of rebirth (Only after receiving prediction from the Dīpankara Buddha as "In a future aeon [world cycle] you will become a Buddha named Gautama." while living as an ascetic named Sumedha ). So, how about other donations, Buddha have donated his wives, sons, daughters, eyes, also their own life numerous times to complete only the Dāna-Paramita. When we talk only about Buddha's giving of children as servants (as thinking "I may become a Buddha by this good Karma" without any angry mind), if all hand tied strings (by others) collected it will become a huge mountain. There is a nice story as 'Sivi-Jathaka' in 'Jataka Tales'. In this story King Sivi donated both of his eyes.