Techniques for improving pronunciation
For beginner level:
I have some children's textbooks with short stories / poems (around 20 - 50 characters). Chinese children in years 1/2 spend time practicing by reciting these short stories until they get them perfect.
Here is an example of the first lesson from the school text book:

一片一片又一片，两片三片四五片，六片七片八九片，香山红叶红满天
Yīpiàn yīpiàn yòu yīpiàn, liǎng piàn sān piàn sìwǔ piàn, liù piàn qī piàn bājiǔ piàn, XiāngShān hóngyè hóng mǎn tiān

Try reciting these until you get fast and clear.
Something else that worked for me was listening to music. You can easily get songs with lyrics from the Internet. Aim for pop songs where the singer has clear pronunciation.  Then practice singing along.
For intermediate level:
I suggest regularly practicing reading aloud. This may sound overly simplistic, but if you get a book that has pinyin so that you can read at a reasonable pace without getting too caught up in the meaning. This will help you exercise your mouth and will improve your pronunciation.
For advanced level:
Try searching the net for some Chinese tongue twisters like:

吃葡萄不吐葡萄皮
Chī pútáo bù tǔ pútáo pí