There are a number of polyfill scripts written that will add placeholder support to browsers that don't natively support the technology.
Rather than repeating what's been written elsewhere, I'd recommend going here: https://github.com/Modernizr/Modernizr/wiki/HTML5-Cross-browser-Polyfills and scroll down about 1/3 of the way to the section entitled Web Forms : input placeholder for a few different options (both native JS and jQuery). Since that entire page is curated by the HTML5 Boilerplate team you can be fairly certain that the scripts provided are of decent quality.
Edit: just saw your comment about not using HTML5. The scripts on the link I provided should still work even if you're not using the HTML5 doctype (no guarantees expressed or implied, etc.).