There are two ways you can do this.

Manual Ping

The manual method of pinging Google is to simply go to http://blogsearch.google.com/ping and enter your blogs URL everytime your blog is updated. This is the straight forward method but many will prefer the Automatic Ping method listed below.

Automatic Ping

If you want WordPress to automatically ping Google everytime you update your blog, then you will need to add http://blogsearch.google.com/ping/RPC2 to your Update Services box in your WordPress admin area. This is found in the options/writing section of the admin area and by default Pingomatic (http://rpc.pingomatic.com) will already be in the text box.

While Pingomatic already pings a number of services for you, by adding Google’s ping url to the Update Services list you are sure to increase your blogs exposure on the net. For more information on ping lists and a comprehensive list of update services see the WordPress Compressed All Inclusive Ping List.