Sitemap should be updated whenever you add a new page , update a new page.
No need to submit it again.
Check Last Downloaded column and you can see it changes quite frequently.

How to submit a sitemap xml to google?

Go to below URL
http://www.google.com/webmasters/sitemaps/siteoverview

If you have a Gmail account , no need to create a new account. You can login to google sitemap back office using gmail account.

If you don’t have a Gmail account ,create a new account .

After you create account , log in to Google back office
There is a way to authenticate the site by uploading google generated blank html.
Now you can fill relevant fields and submit your sitemap. (Before submit your sitemap , you should upload it to your root folder).

How to submit a sitemap xml to Yahoo.?
Yahoo has introduce the yahoo site explorer to submit sitemaps.

Go to http://siteexplorer.search.yahoo.com/
Create an account if you don’t have yahoo account.
Now you can fill relevant fields and submit your sitemap. (Before submit your sitemap , you should upload it to your root folder).

There is a way to authenticate the site by adding meta tag generated by yahoo.
How to submit a sitemap xml to MSN?
MSN doesn’t have separate back office for this. So just use below url and submit the sitemap xml

http://beta.search.msn.com/docs/submit.aspx