There are many ways to accumulate merit. One way is through mantra recitation. Other ways include, releasing lives of animals captive and destined for slaughter / death. Eg. rescuing insects drowning in water is also considered releasing lives. Offering lights/lamps, reciting sutras, doing prostrations, circumambulations, printing sutras, creating buddha images (whether statue or posters etc) , making tsa-tsas, giving food, clothings, necessities to the poor, helping those who are sick, giving medicine, giving alms to the ordained, monks, nuns etc, offering to the Triple Gems (eg, flowers, water, food, incense etc), clearing roads, building bridges, developing patience, loving-kindness and compassion, practising filial piety, teaching the dharma, helping others on the path to liberation, rejoicing in others' merits, keeping vows, taking one-day fasting precepts, doing service for your teacher, temple, sangha...
In general, one should help sentient beings in any way, that will generate merit. But help with wisdom and not with attachment. Also help given should not damage oneself irreparably. The first five of the six paramitas of the bodhisattva path are all about creating merit. Eg, The first of the paramitas is generosity. Amongst generosity, there are 3 types of generosity: giving material needs, giving fearlessness (eg in life release, one is removing fear from the animals about losing their lives) and giving dharma. They are increasingly meritorious because of the increase in benefit to beings both in terms of time and extent of benefit.
If the need is greater, then the benefit is greater and thus the merit is also greater. For eg, giving to disaster relief, people in starvation is much better than feeding a rich man although both are giving and will generate merits.
Merit is dedicated generally to all sentient beings so that they will attain Buddhahood. When we dedicate merit, it is to put the merit to work to the highest purpose of all, so that we don't waste the merit. As long as the purpose is not fulfilled, merit will remain and continue to increase towards that. And also because this merit is locked in an aspiration for all beings to attain the highest perfected state, it will never diminish, just as a drop of water (your drop of merit) is merged into an ocean (all sentient beings' merit) can never dry up completely.