Melatonin does nothing for me. Herbal remedies seem to work well for many people, but doesn't touch me.
I have my bedroom designed for day sleep. I first have a thick, black curtain on my window that shuts out all light. I have velcro attached to the curtain (really a roll) and to the sides of the window. When it's down, it's night-time. Then, when it's not needed, it rolls up to the top of the window and secured there (black garbage bags work well too). Next, I have my ceiling fan, which I use as my "white noise" maker. I'm a firm believer in white noise. Some stores like "Bed Bath and Beyond" have white noise machines that generate multiple sounds. Waterfalls, raindrops, birds chirping, etc. No caffeine after 0600.
Sometimes, when my brain is clocking at 1000 miles per hour, it's very difficult to get to sleep, even with my room set up. At those time, I usually put on something like the "Golf Channel". Peaceful vistas with the announcers talking in hushed tones; or easy rock on the radio.
And sometimes, nothing seems to work. Fortunately, I have a running script for Flexeril after having an L5 partial discectomy. Ten hours later, I awake with no after-effects such as you get from anti-histamines. I use this very sparingly due to tolerance build-up with daily use.