I hail from India where I work as a voice and accent trainer with a BPO.

My work entails teaching people english accent (british and american pronunciation) removing thier mother tongue influnce or you can accent accent neutralization.

Indian speakers mostly posses problems with a number of consonant sounds: S/SH. S/Z, CH, ZH.

I would like to ask the tactics and strategies which can be deployed to remove these errors.
ZH sound is tough to make for adult indian speakers as this sound does not exist in Indian language.

I would like to take your expert coments of how to devise new tactics and strategies to correct on all the above mentioned problems

Thanks a bundle