As I observed myself it looks like I never stop on planning to buy new things that I dont have yet.
The best way to be contented with what you have is to compare it to others who dont have anything at all except for the clothes on their back. I might not have a lot in terms of money and belongings, but compared to a homeless person, Im a millionaire! You can also compare yourself with people in other countries who are in the middle of a war, who are unsure whether they would be able to live or die the next day.
While I do not like comparing myself to others, i see the point of this piece of advice. Apart from that, we must practice gratitude. Only then can we start feeling happy and content.