Krishna is very kind to the meek and simple, so dullness of the material world and consciousness about Krishna go well together. Too much intelligence in material activities are detraction from Krishna Consciousness. Better to remain dull and simple as you are, and thus make advancement in Krishna Consciousness.
Letter to Madhavi Lata, 20 June, 1968
If you are willing to offer your medical services to my students, when it is required, that will be very nice. It is important to keep the body fit and healthy so that we will not meet the obstacle of ill health while serving Krishna. Ill health may hinder one's service, so, we want to avoid it as much as possible.
Letter to Dr. W.D. Currier, 19 June, 1975
To be more clear, the disease is there. Just as the Small-pox disease is there. One has to become diseased, being infected. Similarly, the forms of the body are there. One has to infect the form and it develops. karanam guna sango'sya sad-asad-yoni-janmasu. Because he is infecting different diseased forms, therefore he is developing different forms of body. There is no such case of Small-pox changing into Malaria. Small-pox is Small-pox, and Malaria is Malaria. By the desire of the living entity, he becomes infected with a certain disease or body. Krishna Consciousness is the vaccine for all the diseases (different forms of material bodies).
Letter to Dr. W.H. Wolf-Rottkay, 18 June, 1975
Regarding the tour of North Dakota, this is a nice proposal. So if it is practical, it will be very nice for you and your wife. Only thing you have to do is rent a station wagon, so you can travel, sleep and cook there. Keep with you four or five pairs of cymbals, one harmonium, one mrdanga. One of you play the harmonium, one of you plays the mrdanga, and some persons in the audience can play the karatalas. In this way you can perform nice kirtana.
Letter to Hamsaduta, 17 June, 1969
So there is no question that one activity is more important than another or that Deity worship is more important than Sankirtana, but one individual may be able to perform one activity more satisfactorily than another, so to him that activity will be more important. But in general we cannot say that any of the nine processes is more important than the others, except that if hearing, chanting and remembering are there, that is the most vital consideration for the general class of men in this age. Service to the Deities, as you are asking me, begins whenever you remember Them and offer all your services by remembering at the same time. All activities, words, everything should be offered as service to the Deities, and this offering with remembering will gradually increase as you practice it.
Letter to Prajapati, 16 June, 1972
You remain the beautiful maid servant of Krishna. That is your business, and you should dress yourself always very nicely so that Krishna by seeing you will be pleased. Don't try to be ugly before Krishna. Krishna does not like ugly gopis. We are transcendental artists, musicians, writers, so everything should be beautiful for Krishna. After all we are members of Krishna's family, just like Krishna had 16,000 wives and each wife has thousands of servants and maid-servants and all of them are very beautiful for serving Krishna and His Queens. So the servants of the gopis and queens cannot be ugly, they are as beautiful as the queens. In the Vaikuntha world there is no need of serving anything because everything is already clean and beautiful. It is so clean that just like a mirror when the maid-servants would sweep the floor they could see the reflections of their bodies. So remain always compact in Vaikuntha yajna, simply by thinking of the Glories of the Lord.
Letter to Himavati devi dasi, 15 June, 1972
Our society requires millions of dollars for propaganda work, but Krishna has made us financially poor. I think it is good to remain financially poor because always we shall be able to pray Krishna, asking His help to execute His service. If all of a sudden we become very strong financially, Maya may dictate us for sense enjoyment, and we may fall a prey to her tactics. Therefore to remain poor is one of the qualifications for advancing in Krishna Consciousness.
Letter to Himavati devi dasi, 14 June, 1968
Yes, you can have the band play there in the temple as you are in great need of money to carry on there. When we are in need of money to carry on Krishna's work, then we can resort to whatever means there is available. So long it is always for Krishna and never for personal sense gratification. So you can try to get some money in that way.
Letter to Subala, 12 June, 1968
The idea is that spirit-soul is eternal and the material body is temporary, or for the sake of sense gratification. The mental plane is also material. People being forced to stay on the material platform, they are planning for temporary happiness, and being frustrated, they are jumping from one platform to another which are all temporary. But, fortunately, if one becomes situated on the platform of Krishna Consciousness, then his jumping over material platforms becomes ceased.
Letter to Professor Stillson Judah, 11 June, 1975
It is pleasing to me to here you are happily engaged in devotional service and living as an ideal grhastha. Please go on as you are doing: keep your business separate and wherever you remain keep your family in Krsna Consciousness always chanting Hare Krsna and observing the regulative principles. Cultivate your business for Krsna, remain happy in Krsna Consciousness and always serve Krsna.
Letter to Krsna dasa, 9 June, 1974