How many times should you repeat a keyword in say every 100 words?
I heard repeating a keyword too many times can actually have a bad affect (spamming).
I have never used an algorithm to decide on this but giving it some thought I think 2-3 times is max. It really depends on your sites total word count. If a page has 800 words then you could probably have a keyword repeated 10 times but if you only have 100 words then you just might only want 2-3 times.
Good question overall though. Maybe others have a more authoritative answer.
This method is pretty ineffective now, and is mainly used for targeting google adsense ads. I don't recommend you use a keyword more than 4-5 times per every 100 words. Its also hard to read something which repeats the same things again and again, so you'd lose traffic that way aswell.
Just make sure you have the keyword (or keywords) at least two or three times in the whole text (and not much more than that unless if its a really long article).
Don't bother. These are seen as black hat methods of getting a better ranking, and could be bad for your site in the long run. Just focus on content that users like and make sure it is of high quality and completely unique.
Depends on the site owner and the thread word count.
3-4 maybe.
Don't worry about density try to write content natural way. 7 to 8 percent keyword density is good.