I think a good way to go about it is to do it gradually - like you've swapped your voice's gender and are aging it at the same time. I tried to start out with a seventeen year old guy - Itachi, from Naruto - but found that it's better to start out trying for a younger sound. I worked for it, making sure I sounded like a young boy rather than tomboy-ish girl, and after I was confident I had it down pat I tried for a pre-teen. I repeated the process, and then tried for a regular teenage boy voice. I'm fifteen, myself, so I won't be able to hit any age above young teenager until my own voice fully matures. Like others have said, singing as low as you can and speaking as low as you can really makes it easier to hit those lower vocal pitches.
A really important part of it is tone/texture, too. While pitch makes it, if you still have a girl voice that's just unnaturally low, it sounds like you're sick or... on testosterone or something. xD By practicing a less smooth, more gravelly/rougher voice, it gets easier to sound more masculine even if you aren't at the right pitch. True, not all guys have rough voices, but by making it rough and then smoothing it a litle bit, it sure will sound more masculine. It helps to record yourself, too - if you record, then wait a while and listen, you can sort of figure out how you sound and what to do to make it more guy-ish.
Hope that helped somebody. XD