When I was younger I didn't have much access to TVs and computers, so I would look through magazines and catalogs in hopes of finding beautiful pieces of pictures to cut out and save to one day create a collage. I never really made one, but I did end taping and gluing interesting things to my high school planner and just keeping some interesting pieces in a folder.
This reminds me of Philippians 4:8: "Whatever is true, whatever is honorable, whatever is just, whatever is pure, whatever is lovely, whatever is commendable, if there is any excellence, if there is anything worthy of praise, think about these things."
I had aimed to keep what was beautiful, and this verse asks us to keep what is beautiful in our minds. Truly beautiful things. Not necessarily what our culture thinks is beautiful. But true beauty, as defined by God. As God is, and shown in his work in Creation.
When we go through life, we can try to save in our minds the beautiful things we see. The sun is setting just when we happen to be out, and there's an interesting shade of pink and blue forming. The uniqueness of a person we know well that makes us smile.
We can store these up each day like I was going through a magazine clipping out all the nice colors and pictures. We may not remember them as long as papers last in folders, but it is as easy as that. People often feel like it is hard to think of good things, but it's just keeping in mind to look for those things, and taking a moment to reflect on each one as they're noticed.
After we notice beautiful things, the next step can be to thank God for them, or think of how God has blessed us, or even really nothing. Quietly enjoy that time.
I want to also say that sadness is a part of life due to the Fall. So we can't pressure ourselves too much to reflect on the beautiful always. God calls us to other things as well, like truth and honesty (which are beautiful, but sometimes reveal the ugliness of our condition). We really have to be honest about our condition—the falliness of it and the world around us--to truly see our hope in Christ and also the sheer beauty in even the smallest things. A Fallen world that still has beauty in it, thanks to God. That is more than we deserve (the wages of sin is death - Romans 6:23). So we praise God for his helping us weak creatures.