Investment

Is an NRE account preferable. any ways to decrease the depreciation.is the best way to transfer in bulk when rates are good?

There are two types of accounts which you can open NRE OR NRO.
NRE: The NRE account is an Indian Rupee denominated account. The income is deemed to be earned outside India. Indian Rupees cannot be deposited into this account, only foreign currency can be deposited which will be converted into Indian Rupee at the time of deposit.

NRO: An NRO account is usually opened to manage income earned in India such as rent, dividends, or pension from abroad. The account holder can deposit and manage accumulated rupee funds conveniently through an NRO account. Foreign currency deposited into the NRO account is converted into Indian Rupees. Any NRI can open a NRO account.

I am not sure about others, but i usually transfer in bulk when the rate is good.

will the NRE account be accessible from India or can be changed to resident account? just in case i decide to return after a few years.

Yes NRE account can be changed to normal savings account once you go back.