– Keep it short
One to two paragraphs is good enough. Email with three paragrahs maybe too long for the subscriber to read.
People make subscription on your newsletter and like to receive useful and relavent information. Newsletter should provide help/tips to the subscriber and not only product promotion.
– Subscriber-only Offer
You may provide subscriber-only offer in the newsletter. This can attract people to subscribe and retain in the subscription list.
– Regularly but not too frequent
Send email every four weeks or monthly. This is reasonable and also expectable for the subscriber.