One good exercise to improve pronunciation is to work on tongue agility by repeating the phrase, "Adults who play table tennis are extremely fit." For lip strength, concentrate on purposefully bringing the lips together, and use a phrase with multiple "b" and "p" consonants.
Practice tongue twisters to improve the pronunciation of consonants, such as, "Red leather, yellow leather." Some tongue twisters are helpful for improving vowel sounds as well. To work on the resonance of sounds made with "m" and "n," repeat a phrase such as, "Swimming is fun in the summer months," and spend extra time humming the sounds. Record yourself saying these phrases and other difficult words, and play them back to hear any words or sounds that are not clear. Alternately, practice with another person you trust to give constructive feedback.
Slow down while practicing, as going too fast and mumbling or slurring words defeats the purpose of the exercises. Focus on clear pronunciation, and speed up as you improve. Do not blend consonants into other words, but do not overemphasize them if they start or end a word. Repeat these and other diction exercises over and over to build agility and improve clarity. Consult a specialist before attempting to improve on your own to ensure optimal results.