Quite agree with Palmtree's recommendation.
The two links in BJ#2's for phonetic transcription practice are really useful -- keep practising the consonants and vowels will be good excercise for mouth musceles -
Below are some some words on mouth muscle practise (I don't have the source any more, sorry), but I didn't excersie it at all. Just for your reference (as said before, if you've mastered the English vowels/consonants, your mouth muscles are already well trained).
1、口的开合练习
张嘴象打哈欠,闭嘴如啃苹果。开口的动作要柔和,两嘴角向斜上方抬起,上下唇稍放松,舌头自然放平。做这个练习,克服口腔开度的问题。
2、咀嚼练习
张口咀嚼与闭口咀嚼结合进行,舌头自然放平。
3、双唇练习(这个练习还有助于女孩子美唇啊,呵呵)
双唇闭拢向前、后、左、右、上、下,以及左右转圈
双唇达响
4、舌头练习
舌尖顶下齿,舌面逐渐上翘
舌尖在口内左右顶口腔壁,在门牙上下转圈
舌尖伸出口外向前伸,向左右、上下伸
舌在口腔内左右立起
舌尖的弹练,弹硬腭、弹口唇
舌尖与上齿龈接触打响
舌根与软腭接触打响