What are the basic measures one should take to ensure user privacy to a reasonable extent in smartphones.

I understand the value of privacy, but I am not an expert on security to suggest the measures to be taken.

Lot of non tech savvy people in my family/friends are using all sorts of apps without knowing the risks. I would like to help them understand and ask them to implement the suggestions from this thread.

Please provide your suggestions for ios/android smartphones.

Thanks.