Proper way to address a bhikkhu? I have seen many ways to address a bhikkhu, and I'm wondering about the proper way to address a bhikkhu, in writing?
Bhante is the preferred mode of address if you are addressing the bhikkhu respectfully; note that it is masculine, so for bhikkhunis, Ayye is correct (mostly they use ayya, but I don't think that is technically correct). In English, Venerable x and Reverend x would also be suitable expressions of respect.

If you are addressing them as an equal or as an outsider to the religion, Venerable x and Reverend x would still be proper, I think, but even Brother x would be okay. In Pali, words like bho and ayasma were also used, meaning something like "good sir" or "friend" by those who didn't hold the bhikkhus in any special esteem.

In writing, using the nominative endings (e.g. o for -a stem names) is not technically correct when addressing the bhikkhu in the second person, e.g.:


  Dear Venerable Yuttadhammo,


should really be:


  Dear Venerable Yuttadhamma,


In Thailand this distinction has been pretty much lost, whereas in Burma and Sri Lanka they prefer the a endings for all references.

In Pali, when referring to someone in the third person, you would use the nominative endings, but in the case of corresponding in English, the dictionary stem form is certainly acceptable, as it is with 'buddha', nibbana', etc. Just as we don't say:


  The Buddho was staying in Savatthi.


we really shouldn't be saying:


  Venerable Yuttadhammo was staying in Florida.


As for regional honorifics, e.g. Ajaan (Ajarn, Ajahn) in Thai, Ashin in Burmese, Hamuduruvo in Sinhala, Lakhun in Khmer, etc., I would suggest they only be used by members of the specific ethnic community; I wouldn't think them appropriate for non-members to use to refer to members (e.g. Westerners calling Thai monks Ajaan) unless speaking in the language from which the word comes. In practical usage, however, Ajaan for example has become so widely used by Thai people (even for monks who clearly aren't their or anyone's teacher) that they often insist that non-Thais use it to refer to Thai monks, which of course would be the wise choice when among such people.