Just use some of the keywords in title. You can not stuff title with 10 keywords. Just make title with keyword your after and describe site with it.
Density by itself does not have any meaning... try to check out the top pages within your niche and see what density they had and start from there
There is no optimum Keyword density for title, but there is optimum number of characters. Title should contain the main slogan for the page. All pages should have different title. Also reflect the title on your META description and keywords. But exception I seen is craigslist. There title is very very long. Its PR 8.
Use it in the semantic sense: It's not an advert, it's the title of your page. If you have relevent content on your pages, your titles will automatically be relevent, since a title should reflect the content of the page, not just crammed with keywords. Check out http://www.coffeesh0p.com/shop/ for an example. Simple, elegant, gets the job done.
It is recommended that you use the "keyword phrase" only 1 time in the title. But if you then add 200 additional words, your density is way down. If you only use the "keyword phrase", then your density is way high. My rule is simple, Google uses 64 characters in the SERP's "title" so using more is just a waist of time. But you must left justify the "keyword phrase" to be competitive. If you use all 64 characters and use the "keyword phrase" twice, Google will look at this a Spamming. So, left justify the phrase and use it only once. Keep the associated words down and don't worry about the title density. Just make it read as "an advertisement" of the page. This also holds true for the description, ALT, Title and H1 tags. Keyword density only comes into play in the body text.