How do you encourage people to be generous?

I did not study Communication, Mass Media etc. in my university time. But I am always fascinated to see how people working in the advertising industry can come up with brilliant ideas to persuade people to buy the products of their clients. Times and again, we receive appeal letters from charities such as UNICEF, Orbis, Oxfam and Community Chest etc. Again, I would put myself on the shoes of the authors of these letters. How am I going to persuade the recipients of these appeal letters to open up their wallets and send me a cheque?
I think it is impossible to do a cost-benefit analysis to persuade people to buy a product or to donate money. You cannot just reason rationally with their brains, listing the advantages of using this product rather than that, citing the amount of money you will save etc. The advertising people know very well how to trigger people's wish to spend money by appealing to more primitive and powerful instincts, viz. beauty/youthfulness, sex and money/power etc. That is why in the study of ethics, utilitarianism theory is losing appeal. Cost-benefit analysis may render some justification in taking this action instead of that. But it cannot dictate the final decision.
Due to their job nature, religious people have little choices available to persuade people to do good. Unlike their counterparts in the advertising industry, pastors, imams, monks and popes cannot appeal to money, power and sex to persuade people to lead a virtuous life. They can only appeal to the afterlife. Do evil and you are condemned to the eternal fire in hell. You had better do good. Fear seems to be their primary weapon. Of course, it is not the only path. Many other brave souls have explored other means to persuade people to do good. In the Second Epistle to the Corinthians, Paul told the readers to honour their promises to send alms to Jewish brethren. He appealed to their sense of honour.
for I know your readiness, of which I boast about you to the people of Macedonia, saying that Achaia has been ready since last year; and your zeal has stirred up most of them (2 Corinthians 9:2).
Whether the Corinthians were truly eager and ready to donate or not, Paul said that they were. Moreover, Paul was responsible for publicizing their generosity among the Macedonians. Back to the Corinthians, Paul told them that their eagerness had in effect, stirred up the generosity of the Macedonians. Paul was not telling lies. Generosity did inspire generosity among others. The ingenuity of Paul was that he pitched the Corinthians against the Macedonians, creating a virtual rivalry that generated greater contributions from both sides. The rest of the argument does not seem to be crucial.
The point is this: he who sows sparingly will also reap sparingly, and he who sows bountifully will also reap bountifully
Each one must do as he has made up his mind, not reluctantly or under compulsion, for God loves a cheerful giver (2 Corinthians 9:6-7).
Be generous for God will reward you generously in the future. Who cares? This piece of advice is effective only if people believe in God and/or in an afterlife. Otherwise, it is less effective than using honour as an incentive.
Of course, Paul could not force the Corinthians. They must donate voluntarily. Otherwise, their generosity would not be counted as a virtue and neither would it win the Corinthians any (spiritual) benefit. The whole project would be ruined.

Take the case of the recent controversy created by the relocation of the rehabilitation centre for young drug addicts. There is a conflict of needs. The College and the society need a bigger premises. The local residents need a secondary school. The government needs to appeal to the generosity of the local residents in order to relocate the College. But how could the government persuade the local residents to be generous by sacrificing their secondary school? Most of the local people might believe in an afterlife but they do not believe in God. Therefore, it would not be effective to appeal to the spirituality of the local residents. If in the end, the government forces the local residents to be generous, she will create more resentment. The whole project would be ruined. Can the government give them honour in exchange for their secondary school? This is a viable option.

Dear Lord, I know You will give the College a perfect site. I pray for the government to come up with a win-win solution. I pray for the local residents of Mui Wo that they open their hearts wide for the benefit of the society at large and for their own good. Amen.

Appendix:he who sows sparingly will also reap sparingly, and he who sows bountifully will also reap bountifully.
ὁ σπείρων φειδομένως φειδομένως καὶ θερίσει, καὶ ὁ σπείρων ἐπ' εὐλογίαις ἐπ' εὐλογίαις καὶ θερίσει. (2 Corinthians 9:6)
One man gives freely, yet grows all the richer; another withholds what he should give, and only suffers want.
A liberal man will be enriched, and one who waters will himself be watered.
εἰσὶν οἳ τὰ ἴδια σπείροντες πλείονα ποιοῦσιν, εἰσὶν καὶ οἳ συνάγοντες ἐλαττονοῦνται.
ψυχὴ εὐλογουμένη πᾶσα ἁπλῆ, ἀνὴρ δὲ θυμώδης οὐκ εὐσχήμων (Proverbs 11:24-25).

He scatters abroad, he gives to the poor; his righteousness endures for ever.
Ἐσκόρπισεν, ἔδωκεν τοῖς πένησιν, ἡ δικαιοσύνη αὐτοῦ μένει εἰς τὸν αἰῶνα (2 Corinthians 9:9b, Psalm 111:9, LXX, 112:9 MT).