After you’ve chosen your niche blog topic, it’s time for the domain name. Using the Google Keyword Tool, please refer to your niche keywords. You want to search to at least 10,000 Now you have to look for a domain name that uses these keywords. If you have a good formula for your domain, that is even better. But do not buy the domain yet.
I basically choose the bravenet for this purpose i have a free account there now when you logging they have a page for domain registration where you can get the url suggestions.I have carefully watched that their suggested urls are really good than any other services so i just put the keyword in it and it provides me the related url names which i copy and get registered with my desired registrar.Put your desired domain names in Google in quotes. See how many queries come. To search under 200,000, so you can be placed on Google. Also check that the top two places by the website. If so, select a different domain name.
I am not advertising this site this is informational post only.
Go to Bravenet.com
Login in it
Click on Domain tab >>> You will find this page 
just enter the keyword which you have picked from the keywords tool of google and choose the most matchings name.
how to get best doman was a useful article
It appears that success of the website correlates with some properties of the domain name itself. Here is a link to an interesting research:
m.y.