How to have a communication channel open with your kids??

Hello Everyone!! Hope all of you are having a wonderful week . Mine was a usual one and a little hectic.

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I recently read an article that was shared on FB on  “Why it is important to teach kids “How  to say “NO””.It talked about how teaching kids to say no will help them when someone forces them to have a drink or try smoking or even drugs.The author talked about the scenarios and what answers she discussed with her  teenage son for various situations. Read the article here. Thanks to my friend for sharing this article.
After reading this, I realized that  my problem is not about teaching this, my problem is /will be on how to make my son talk to me about the important things happening in his life. I have seen pathetic situations where parents will be the last people to know about things that their kids are involved in.
 I have talked to some of my friends and do the following
*Tell my son about my day and ask him about his. Sometimes works , sometimes he would just say “I dont know what I did”.I know he is only 31/2 years old and cant expect too much from him but I am still trying
*I would ask him to share a secret with me that is only between me, his dad and him. Sometimes this works and sometimes he would just say “I dont have anything to say”.
*I would ask him about sad as well as exciting things that happened at school and he would sometimes tell me that .
*I would like to talk to his friends parents to know if their kids talked about what my son does at school and so on.
I know as he grows it will be more challenging, and I  just have to enjoy the cute age he is in and not worry about it.Since me and my husband work full time and we devote only a fixed amount of time for the kids everyday ,  we always try to make it more quality time and interact with them to know more about what they did and what they like and dislike.
On a side note media and cell phone will be another big challenge once they get their own phone it might get even more complicated.
This is why I feel parenting is so hard. Every phase has its own challenge and no one is perfect. We all try to do our best.If any of you feel that there are some things that we can do to start early and make our kids talk to us more about what they do at school please do suggest it here.