Can you explain what you mean by unnecessary downloads?
There's no point in having someone on their cafe wi-fi at however-many-euros-per-kb downloading 300kb of scripts, a few 1MB background images and whatever else that they're not even going to display anyway (because you're using media queries to hide stuff on smaller screens). The mobiles will download all the stuff you reference in <link> tags, and if you do it wrong, ppl on crappy connections are going to feel like they're on dial-up trying to access your site.
One way people prevent "unnecessary downloads" is, they let the server sniff the user agent (which is tricky at best, as browsers lie about which user agent they are... and your server will then need a list of either mobiles or desktops so that it knows which type of content to send) and sends a version of the requested page to mobiles that simply does not have the scripts and images etc.
But another way is to build backwards: make a simple, working version of your page, and start with a basic stylesheet that styles for mobile/smaller screens.
Now I have to check ppk's site (quirksmode.org, where he's been doing many tests on mobile browsers), but browsers will make a request for a background image even if they don't display it, IF they downloaded a stylesheet. What I'm not sure of is, will they download the stylesheet (but not run it) even if it's for a media query they do not match?