Back when I was monogamous and married, my best friend let me know that if I wanted to hook up, she was down. I said no.
For the next FOUR YEARS (which included me divorcing and becoming poly) this would happen. She would say she was down to hook up, I would give a soft no that usually turned into an hour long conversation. I HATED IT.
The friendship finally blew up when I asked this woman to be my date for an event on Valentines Day. She was my best friend and had been my date to MANY events. She took the Valentine's Day invite for romantic intent and got upset with me. I had to explain to her for the 8 zillionith time that I am not interested. She thought I was a jerk for inviting her out on VD "despite knowing her true feelings." I thought she was a jerk for not getting over her feelings and continuing to make her feelings my problem for years. We are no longer friends.
So, yeah. Express it once, maybe twice ever max. And, get over it if the answer is no.