Communication Arts' website is a good place to start: www.commarts.com/

Look at how other illustrators structure their portfolios. Consider craft, content, pacing, and flow. Develop a narrative with your portfolio that starts with one of your strongest pieces, flows through a few other exceptional ones, and ends with a bang. Less is more - only pick pieces that you can stand behind, thoroughly explain, and give an idea of both your particular style and your ability to execute on concepts.

For publishings specifically, you may want to check out mediabistro.com

Do you have a website? An artists rep? The postcard samples that illustrators send out are hit or miss as well - some of them I keep and I've even given one or two assignments based on them, but really, many art directors when looking for an illustrator, are doing so online late at night relying on the mysteries of search engine results, bleary eyed and praying to find just the right style. Or maybe that's just me.