Good VO Exercise: Speak with a Pencil in your Teeth!
(Exercises republished from VoiceOverVoiceActor.com/Take-Action)
To help improve your diction, lightly grip a pencil (horizontally) between your teeth, so that you are forced to really work your tongue to create the sounds.
Practice speaking out loud the following 3 tongue twisters, clearly enunciating each word:
A thin little boy picked six thick thistle sticks. Sidney Shelly flies through thick fog. This thatched roof is thick.
After saying them each several times, remove the pencil and try saying them again. Listen to how much the clarity of your speech has improved.