For a time I worked as a radio announcer and newscaster. I prepared for it by taking a course in broadcasting, which included several weeks of pronunciation, broadcast English, and diction. Unfortunately my books and notes are packed away right now and I'm not sure where they are, otherwise I would post a bunch of really good diction exercises for you (in fact, I should still be doing them myself, because they keep me sharp). I'll keep looking for them, and when and if I find them, I'll post them.

Check your local library for books on broadcasting, acting, and singing. There's probably a lot of stuff out there. In the meantime, googling through cyberspace turned up THIS. I don't think they're quite as intense as the one's I used, but they're pretty good. Read them aloud several times a day. Relax your throat (yawn), use good breath support, and read them slowly and precisely at first. In fact, over-pronounce the words—exaggerate them. Once you can get through them without goofing, speed it up a little, still keeping your enunciation sharp and precise and still over-pronouncing (also, look up the correct pronunciations of any words you're unsure of).

My diction was fairly good to begin with (I'd taken a lot of voice lessons), but working on exercises like these really sharpened things up. In addition to helping me get broadcasting jobs, they improved my singing diction as well. I hope these help.

Don Firth