I have a deep voice too, and broadcasting is my family trade so I've always been interested in the voice. I had the same problem as you -- deep voice but mumbling and people having trouble hearing what I was saying.
The great news is that since your voice is naturally deep, once you get it in shape (esp when you are older) the girls'll be rubbing themselves up and down your leg. But in fact anyone can improve their voice a great deal, no matter how high-pitched it is: most bad voices are the result of habits you picked up.
Here are some tips that have benefitted me. This is a random compilation of the most beneficial stuff I've picked up from my voice coach and various books I've read:
-The #1 problem with mumbling is linked to self-conciousness. Try to stop worrying about how your voice sounds, whether what you are saying sounds "right". Don't try to make your voice sound a certain way, rather focus on getting rid of bad habits to find your natural voice. Almost everyone's natural, habit-free voice is very good. Why do you think, as one of my books said, that a tiny little baby is able to make such a loud crying sound for hours on end, without getting tired? 99.99% of us are born with a natural voice, but we pick up bad habits that squander it.
-Pronunciation. Are you pronunciating everything clearly? Try reading a paragraph from a book with a wine cork placed gently between your teeth (just so much that its pretty loose -- dont clench down on it or something). Go sentence-by-sentence, overpronunciating every syllable. You should be trying exagerrate to the point where someone listening to you with the wine cork in your mouth could, with effort, make out what you were saying. At the end of every sentence take out the cork and read the sentence again. This will teach you how to pronunciate.
-Breathing. Your lungs are much bigger than you think. They're not two tiny bags in the middle of your body -- they stretch all the way from near your collarbone down to your belly. There are alot exercises to improve this, but I don't think I can explain them adequately here and I wouldn't want you to hurt yourself. But a good voice coach or book can show you how to increase your vocal power through opening up your breath.
-Body language. This was the #1 thing that improved my voice. This is hooked into habits again. If your body is uptight a tense, your voice will be too. Do you suck in your belly? Let it out! Sink your shoulders if you are holding them up. Relax your body and your voice will be more relaxed as well. I have found, since I started paying attention, that if I am distracted for a few minutes I will find my shoulders hunched up and my stomach sucked in, through habit. Relaxing my stomach alone gives my voice much more depth and power.
I hope this helps. I have been meaning to write a longer article on the voice for the Tips section for quite a while -- if you guys are interested let me know and I will get on it.