Having worked in the Jewish Publishing Industry for a number of years, I can tell you that it is common practice to use approbations on partial manuscripts, or even for other books written by the same author, or even just a letter attesting to the author's reliability. There are no real guidelines. I have seen haskamot from Rabbis who passed away before the book was ever even written (either written regarding a previous book or about the author)!
Regrading how to get a haskama: I would recommend personally taking the manuscript to the Rabbi from whom you want to get an approbation, handing to him and reminding him of who you are, how you know him and what your credentials are. I would also recommend including a "cover letter" with your qualifications and other approbations you may have already received. If you cannot give it to him personally, I would recommend sending it in the mail with a cover letter and a picture of you, to remind the Rabbi of who you are.
If you don't personally know the person from who you are requesting an approbation, I would recommend finding a friend or someone who does know the Rabbi personally or has some connection with him to do the hand off.
The key to getting a good approbation, from my experience is the Rabbi knowing who you are or getting a word of mouth referral.