What can I do to prevent my child from wanting and buying unnecessary toys whenever we're at the mall?

For me, before I go out I will set my boundaries with kids first: i.e. telling them today we are not going to buy any toys and you can list down the toys that you like in your wishlist book if you happen to see something that you like and I will buy it for your during your birthday. My kids normally understand and so far there is no tantrum happening at toys' shop. Another thing is refrain yourself from buying too many toys for your kids. It is good to teach kids how to appreciate and treasure on what they have on hands and you can rotate the toys every month instead of keep on buying new toys.