Not sure there's a trick. I have a Canon Pixma 5000...
From iPhoto I select "4x6 paper (borderless)" from the "Page Setup..." dialog (File -> Page Setup...), and "Full Page" from iPhoto's Print dialog (File -> Print). I also make sure I choose the correct paper type ("glossy premium", "photo plus", etc...). Hit PRINT, and out pops a borderless picture.
If your camera doesn't take true 35 mm film aspect ratio images (but rather 4:3 like some digital cameras do), make sure to crop your picture via iPhoto to 4x6 postcard or 4x6 portrait first.
YMMV, of course, depending on your printer and drivers, but it shouldn't be that tough I guess.
Just make sure you set the paper size in the Page Setup dialog to 4x6, then Full Page from the Print box (don't reselect 4x6... you've already told the OS you're printing to 4x6" paper, so you now want to print a full page.