You should not use your keywords in every sentence, but use your keywords sensibly. Recomended Keyword Density is 3 to 4% of your total indexable content.
I would say between 2% - 3% is the ideal value for keyword density and is also per the ezine articles guidelines. Dont overuse the keyword in every sentence or it will sound like spammy. Place the keywords appropriately at strategic places in the paragraph.
Keyword density is the percentage of times a keyword or phrase appears on a web page compared to the total number of words on the page. In the context of search engine optimization keyword density can be used as a factor in determining whether a web page is relevant to a specified keyword or keyword phrase.
eagleshot is right, make it natural and don't try to re-phrase. It can give a negative impact and can decrease your daily traffic
Keyword Density is the percentage of times a keyword or phrase appears on a web page compared to the total number of words on the page. Too high a density can result in a web page being classified as spam while too low a density will cause a page to not be indexed as well for the given keyword. It an be calculated as the product of Total number of keywords with 100 divided by total number of words