The technique from the video on this page works for me:
http://www.deeper-voice.com/an-exercise-to-make-your-voice-sound-deeper. Usually the deeper range it gives me only sticks around for a few hours after I do the exercise unless I do it regularly. And I have found that doing this daily has noticeably changed my voice for the better. Though I still sound like a kid... but at least not as girly as before.
Also, if when you speak your voice slips upwards at the end of words, fix it. Guys usually have downwards slide of pitch at the end of words/phrases, so it will stand out if it's the opposite. A fairly monotone intonation overall can also help. Also, work on speaking loudly and regularly, not in short, fast bursts.
I also sing songs in the deepest voice I can manage, which has seemed to expand my lower range a bit in the long run. Lying down when singing, as weird as it feels, forces you to use your chest voice rather than head voice, if that makes any sense at all. It gets a deeper, fuller tone. If you get used to the feel of that you can use it in ordinary situations.