For Firefox, there's a search plugin (https://addons.mozilla.org/en-US/firefox/addon/duck-duck-go-...) that adds DuckDuckGo to the list of search providers.

For Chrome, it appears to be a little more complicated. You can use DuckDuckGo for searches, but it doesn't look like you can alter the source for URL suggestions. Details on how to use DuckDuckGo as Chrome's search provider are here: http://ddgg.nfriedly.com/#oneclick.