Warm up your voice properly. Take singing lessons or work with an acting coach to learn how to do this. Concentrate on breathing into your abdomen and relaxing your jaw, throat and soft palate. Stay relaxed as you speak or sing and your voice should naturally drop in and start to sound deeper.
Use a mic that has a good proximity effect. Opt for mics that are designed for vocal use, as these often provide strong proximity effects.
Position your mouth very close to the microphone. Get as close as you can without your plosives popping on the track. Use a pop filter to help reduce plosive impact even when very close to the mic.
Hold the mic loosely if you are holding it in your hand for a live performance versus recording in a studio. If you grip it too tightly, you can limit your ability to sound low on the vocal.
Process the mix correctly in order to emphasize the low end of your voice when working in a recording environment. Apply an equalizer effect to the track and boost the lower end frequencies. Monitor the changes carefully, though, so that you don't end up with a voice that sounds artificially processed. Subtle shifts are all that is needed to bring out the low end.