I have listened to a two hour sermon on Metta Bhavana by Chanmyay Sayadaw about ten years ago.
The practitioner of metta bhavana is like a musician practising to play a harp. The more he plays the better he becomes. Whether the listener benefit or not depends on whether his attention is tuned to hear or enjoy the music.
Sending goodwill and loving kindness to all beings in all directions gives the person an immense joy and satisfaction.
To practice metta to promote concentration needs a specific technique.
There are some useful ways to prepare ourselves before starting metta bhavana. One should wash and dress clean clothings, preferably eat a vegetarian meal (it is said to help developing metta by avoiding meat of killed animals). It is sensible to follow the basic morality, five precepts. Imagine how a mother would feel towards her only son. Think towards ourselves what we want and need. Prime our metta bhavana by wishing ourselves to be healthy, prosperous, to be free of physical pain and mental distress, to be able to fulfil our own needs and comfort. Then we carry on with sending metta to respectful teachers, sanghas/monks, parents, relatives, friends, workmates, neighbours, people difficult to deal with, and lastly enemies (when the metta state is very strong).
One should start sending metta ( by wishing good health, prosperity, freedom from bodily pain and mental distress, to be able to fulfil own's needs, to realise higher dhamma and to retain the previous wisdom) repeatedly to someone we respect ( like a meditation teacher or a monk), again and again, many many times until you can see or concentrate strongly on that person very clearly and continuously. Sometimes we may see his face (which may be smiling).
We have to make a wish to have a state of mind filled with metta, which has five qualities: vitaka (aiming), vicara (rubbing against object), piti (rapture/joy), sukha (happiness), ekatta(unity or singleness). After repeated practice and when the mind is calmer, we can then wish for a state of metta with 3 qualities: piti, sukha, ekatta and continue reducing these one by one until ekatta state of metta remains.
Once the concentration is very strong, we can try sending to another person from the similar category of people. If the mind become shaky, we have to return to the previous person again to send metta. An example of trying to fill a small pond with water before we can send water along the connecting drains on the ground to other ponds was given. If the first pond is not full, water will not easily flow to the second or third ponds.
There are a category of persons not suitable to start sending metta because it can be difficult, ending in unwanted mental states or impossible to cencentrate. They are the people we love intimately ( can become very sad if they are ill or miserable), those we hate ( can result in strong anger), those we barely know ( difficult to create a metta state), opposite sex ( can cause lust), people who have died ( mind will not settle, because their mental object is difficult to focus). In the commentary a young monk tried repeatedly to send metta to his master without success and later found out that his master has passed away. (It is very difficult to say whether the mind actually searched or reached out to find the object because mind does not follow the usual physical time and space dimensions.)
When the concentration is strong, we can move our focus of metta from one category to the other and will be successful even towards enemies.
Persons who practice metta will have the following benefits: good sleep, refreshed on waking up, good dreams, loved by humans, devas, free from dangerous and poisonous animals, protection from weapons, poisons and fire, good concentration, clear complexion, peaceful at the time of death and reaching the brahma (devine realm) in the after life. (11 benefits of metta)
(P.S. If the initial aim of doing metta bhavana is to support the vipassana mindfulness, the deep concentrated metta state can be observed as a vippassana mind object, and notice annica, dukkha and anatta characteristics.Note: samatha-yanika/vipassana-yanika)
May you be successful with your metta bhavana.
Peter Khin Tun