Proper letter writing etiquette I've recently been writing a lot of letters in Mandarin and I have several questions regarding the form.

Chinese has many salutations in letters. For the most part, is it okay just to use 亲爱? If not, what is most appropriate?
Are Chinese letter structures [eg. Name, Address, Date, Recipient, Body, Closing] much the same as English letters?
I usually end my letters:


孔世茂 [My Chinese Name]
上

Is this acceptable? What are some other endings for letters?
Thanks for your time!
I think those are generally fine for normal purposes, especially if you're emailing. Traditional etiquette has substantially declined with email use. 亲爱 is quite a bit more personal than the equivalent English "Dear xxx" though, so you could replace that with just a greeting, e.g. "王先生您好

But since you asked for "proper etiquette"... Here's a brief rundown of the main points for a proper Chinese letter. Though I've left out some less important sections, so for truly official purposes you'd want to consult a real style guide. 

For obvious reasons hardly anyone writes to actual friends or family in such complicated manner, although government agencies / companies/ wedding invitations and other formal occasions still adhere to these styles.



稱謂: Addressing the Recipient

This section is similar to English letter writing. Use an appropriate title e.g. "President Huang" or "Ms Wong" or "Mother". Do not include names for elders in your immediate family.



提稱語: Inviting the Recipient to Read

You might think of this as sort of like "Dear " in English, except this comes after the recipient's name+title. Choose an appropriate one from below:


Parents/Grandparents: 尊鑒(common), 膝下(common), 膝前, 
Teachers: 道鑒(common), 函丈, 壇席, 講座, 尊前,尊鑒, 賜鑒
Generic elders: 賜鑒(common), 尊前, 尊鑒, 鈞鑒, 尊右, 懿鑒(women only)
Fellow students: 硯右, 硯席, 文几, 文席
Generic equals: 大鑒(common), 惠鑒(common), 台鑒, 左右, 足下, 閣下, 雅鑒
Children/Nieces/Nephews: 知之, 知悉
Generic juniors: 鑒(common), 青鑒,青覽, 入目, 入覽, 收覽, 如見, 如晤, 如握, 如面, 收覽, 收閱, 收讀,  閱悉, 知悉, 見字
Military officers: 幕下, 麾下, 鈞鑒. 鈞座
Politicians: 勛鑒, 鈞鑒, 鈞座, 台座, 台鑒
Educators: 講座, 座右, 塵次. 有道, 著席, 撰席, 史席
Buddhist priests: 方丈, 道鑒, 有道
Taoists priests: 法鑒, 壇次
Generic/Christian priests/nuns: 道鑒
Married couple: 儷鑒
Mourning: 禮席, 禮鑒, 苫次, 矜鑒


And last but not the least:


Modern Age Catch All Replacement: : &lt;--- yes, a colon




申悃語: Ending the Letter

You can either pick an appropriate one based on recipient, or based on the nature of your letter.


Reply letter: 耑肅敬覆, 耑此奉覆, 肅函奉覆, 耑此敬覆, 匆此布覆
Congratulatory letter: 敬申賀悃, 肅表賀忱, 用申賀意
Mourning letter: 恭陳唁意, 肅此上慰, 敬申哀悃
Thank-You letter: 肅誌謝忱, 敬此鳴謝, 耑鳴謝悃
Elders: 謹此, 肅此, 肅此上達, 敬此馳稟, 耑肅奉稟
Equals: 專此, 耑此, 特此, 草此, 耑此奉達, 匆此布臆, 特此奉聞, 草此奉達


Or:


Simplification: skip this section entirely.




問候語: Paying Respects

This follows the previous section immediately. You can pick one based on who the recipient is.


Parents/Grandparents: 敬請 | 福安, 叩請 | 金安
Teachers: 敬請 | 道安, 恭敬 | 教安, 恭請 | 誨安, 敬請 | 講安
Generic elders: 敬請 | 鈞安, 恭敬 | 崇安, ``敬頌 | 崇祺,恭請 | 禔安`
Equals: 敬請 | 大安, 順頌 | 台祺, 敬候 | 近祉, 順候 | 起居, 順頌 | 時綏
Juniors: 順問 | 近祺, 即問 | 近好, 順頌 | 日祉
Military: 敬請 | 戎安, 肅請 | 捷安, 肅頌 | 戎祺
Politicians: 敬請 | 鈞安',敬請 | 勛安,祇請 | 政安`
Educators/Academics: 即頌 | 文祺, 即頌 | 文綏, 敬請 | 學安
Writers: 敬請 |著安, 順請 | 撰安
Business: 敬請 | 籌安、順候 | 財安
Traveller: 敬請 | 旅安
Married Couple: 敬請 | 儷安
Someone ill: 恭請 | 痊安, 敬祝|  早痊, 敬請 | 愈安, 即請 | 衛安


Or choose one based on the season/time:


Spring: 敬請 | 春安, 順頌 | 春祉
Summer: 敬請 | 夏安, 此頌 | 暑綏
Autumn: 並候 | 秋綏, 即請 | 秋安
Winter: 敬請 | 冬安, 敬頌 | 冬綏
Morning: 即頌 | 晨安, 即請 | 早安
Noon: 此請 | 午安
Evening: 即頌 | 晚安
Generic good day: 即請 | 刻安, 順候 | 日祉, 即候 | 時祉


Or based on the purpose of the letter:


Wedding congratulation: 恭請 | 燕喜,恭賀 | 大喜, 恭請 | 喜安
Happy New Year: 恭賀 | 年禧, 恭賀 | 新禧, 即頌 | 歲禧
Mourning: 敬請 | 禮安, 順候 | 素履, 此候 | 孝履, 順問 | 苫次


Note that these are supposed to be broken int two lines; i.e.:


  bala bala bala bala。肅此上達。敬請
  
  福安


Also, notice how most of these reuses the same set of perhaps 3-4 phrases for the first half. For these the situation-specific content lies in the second half of the sentence. (the part that comes on a new line). Which means you can mix the first half a bit for variation if you wish.



署名: Sender's Name

Can be either personal name, or the family name plus personal name. This should be prefixed with your self-reference, in smaller font. The self-reference should complement with the title of the recipient you used in the first section; i.e. if you used Smith 吾師 to address the recipient, then here you'd say 受業 TrustedDeveloper. Other common ones are:


To grandparents: 孫, 孫女
To parents: 兒, 女
To teacher: 生
To friends: 弟, 妹




末啟詞: Finish

This goes directly after your name from the previous section; i.e. on the same line. It is analogous to saying "from " in English. Choose an appropriate one from below:


Parents/Grandparents: 敬稟, 叩稟, 拜稟, 肅稟, 謹稟, 叩上
Generic elders: 敬上, 謹上, 謹稟, 拜上、謹肅, 敬肅, 敬啟, 謹啟
Equals: 拜啟, 敬啟, 啟, 上, 白, 謹啟, 謹白, 手啟, 手上, 頓首, 上言, 拜言
Juniors: 字, 諭, 手諭, 手示, 手草, 草示


Alternatively, when the letter is a reply:


Reply: 復, 手復, 謹復, 肅復