This is a common problem for speakers of southern Chinese dialects like Cantonese. See my web page "http://www.chass.utoronto.ca/french/as-sa/PEP2/phonetics/index.html" and click on "voiced/ unvoiced consonants." Then you can click on the sound files (recorded at 44,000 Hz, nice quality) and listen carefully, repeating, to hear these differences.
This should provide some practice.
- For Teachers