1. You should know German normally, i.e. as a minimum after 1-st year of training. Otherwise it will be not interesting for Germans to communicate with you. And you would began to communicate with someone if to you will write in your letter only "I am a student and I live in city of N." . All over again learn language even up to a base level and only then start to search for friends with the help of letters.
2. If you search for the person who will write constantly and much - search among persons of average and advanced age. The youth writes irregularly, it is not enough and illiterately is I to you I speak as the expert in this business. And people in the age of from 28 years also are more serious - write more - less constantly.
3. Observe network etiquette. Do not hasten to name the person on YOU all over again ask sanctions to it. Respect his sights, and certainly do not abuse him that in him is not pleasant to you. If the not nice citizen has got to you - simply refuse correspondence with it(him).
4. Be proud and do not ask anything (parcels, gifts etc.).
5. Write regularly and write the big letters. My experience specifies, that a normal interval of letters is 1-2 times per one week with volume of letters 3-7 kilobyte. More often - it will be tiresome for you and for your correspondent. More than 3-7 kb - it is too much for the letter. After the first 2-3 letters when you have in detail described who you and than like to be engaged, there is a question - "About what to write further?". Write about the daily affairs, about holidays, about local customs, about a nature, about the friends.
6. Read closely reciprocal letters and necessarily try to reply all given to you. It is very important. Your correspondent should feel, that his letters are read attentively. Ask necessarily also about life of your correspondent. That the person felt, that his life is interesting to you.
7. Write competently. Do the new paragraph if you pass to a new theme.