For most of my life up to age 18, I have been suffering a speech impediment.
I think when I was 4 or so, I had intense hearing problems, so that greatly delayed my communication skills. My mom sent me to the place called CATC (children's activity and treatment center) where I had speech therapy done. If this hadn't happenned, I wouldn't even be able to make intelligible conversations.
Basically, it was limited to mumbling until grade 10. Now, in grade 10, me and my "friends" were playing hacky-sack in the spring of grade 10, and this kid said something like: "That's the stupidest thing I've ever heard, besides your voice!" This motivated me to get speech therapy, for the taunting didn't stop about my voice. I went to the same doctor who gave me my early speech therapy, but she had her own practise now.
I learned a few exercises from her, that I still use today. I will divvy it up into several sections:
1) Mouth and Sound: This helps increase your volume because the aim is to say the sounds as loud as you can, plus you are exercising the mouth muscles, so that you can sound more articulate.
i) OOH-EEH: like it sounds, exaggerating the complete sound of it, making it sound loud, but when saying OOH, stretch your lips so that it looks like you're about to frantically blow out a fire. When saying the EEH, make a big smile, stretching your face muscles as far back as you can, which helps exercise the muscles (speech therapy is about exercising oral muscles). Repeat this 10 times each day.
ii) AHH-EEH: same, except with the AHH, open your mouth as wide open as you can, except if you want a consistent AHH-EEH, stretch your mouth downwards as you finish saying EEH, and open your jaw as much downward as possible for the AHH. Do 10 times.
iii) PATAKA: like it sounds, but this helps you speak fast, because this is the exercise where you build up on your speaking speed. You don't need to stretch your mouth muscles on this one. Do 10 times.
2) Tongue: your tongue is an important part of speaking. This is what determines if you sound clear or if you sound mumbly. These exercises are meant to strengthen your tongue muscles (maybe help the guys with pleasuring their loved one ;)
i) In-Out: when doing tongue exercises, open your mouth as wide as when you say your AHHs. Doing in-outs is basically sticking your tongue as far out as possible with your mouth open (open at ALL times), and pulling it back in, curled up. Do it rapidly with much vigor, repeating 20 times.
ii) Up-Down: hit the tongue at the very roof of your mouth, and then hit the gums just under the back of the bottom row of your teeth. This works, because if you instinctively do a passing back-and-forth motion from the roof of your mouth with your tongue, it will bounce at the right spot. Do this 20 times.
iii) Left-Right: Stick your tongue out as far as possible, and rapidly hit the sides of your mouth 20 times.
The final exercise is for the face. You know just under your eyes, you have that bone part of your face that sticks out? Cheekbones, that's it! That is an important part of speaking, to exercise the muscles in this area. Make a BIG smile, but also try and stretch your cheekbone muscles as high up as possible, also stretching your smile as far back as possible. Repeat 10-20 times, your own choice.
Ever heard of a good British actor/comedian called Stephen Fry? He had terrible speech as a child, and now he's a well-known, very articulate speaker! He had a very loud voice, sounds very clear, and it's as if you could hardly tell he ever had a speech impediment! He was in "Blackadder Goes Forth" with Rowan Atkinson. I still have ways to go (I kinda gave up, but maybe I'll start practising again soon), but with willpower, dedication, and few obstacles, one can get their voice back.
I wish you good luck, it's about time someone wrote about speech impediments here.