Articulation gets a bad reputation from the training methods that everyone sees in the movies. Tongue-twisters like "Sally sells sea shells by the sea shore" are useful, but there's a more straightforward method to practice.
Articulation itself isn't about speaking "correctly" - it's about giving yourself the choice of how to speak. Once your ability is developed, you'll be able to modulate the clearness with which you speak instead of just volume. It's also a great way to learn breath control.
For each of the exercises below, take a deep breath into your stomach and repeat the sounds as many times in a row as possible.
10 times in one breath
This is a great place to start, especially as you learn the IPA symbols and your mouth starts to force itself into positions it usually avoids.
15 times in one breath
This is a good target to shoot for - certainly attainable within a month of practice.
20 times in one breath
This should be your overall goal.
30 times in one breath
You've mastered the articulation and the efficiency of your breath. Try doing the exercise with a cork or pencil between your teeth to make it even harder.
40 times in one breath
Holy shit dude, calm down.
- Get the sounds right first, then worry about speed
- Don't emphasize any syllable too much - keep them all on the same level to save breath
- Breathe into your stomach - if your shoulders rise or your chest expands, ya done fucked it up
- Be efficient - use only as much air as you need
- The exercises are notated in International Phonetic Alphabet (IPA)