I live such a frugal and abstemious life already that I can't really see anything to give up. So, I've dusted down my hair shirt and I'll wear it for the next 40 days.
I need limit my intake of cheesecake, chips, crisps and chocolate but for the next 40 days I promise to refrain from abstaining of the aforementioned foods.
I was thinking of giving up atheism. I've been flirting with the idea of an all-powerful, all-knowing, benevolent super-being who silently protects me from harm and who has my long-term interests at heart.
I might give up disbelief for a while and see if my life becomes noticeably better... I'll let you know what happens. Fingers crossed!