A few thoughts:
Stress is a big and bad actor for me. When I was under extreme stress during my divorce, nothing helped. I would try something like a med and it would work once, then be counterproductive.
My history with things like tryptophan, 5HTP & melatonin are such that I tend to react opposite "normal." I can now take 0.5 or 1 g melatonin and it is OK. If I take much more, it is counterproductive. Sometimes I'll take 500 mg tryptophan, but any more is bad news.
Staying up too late, in my case after 11 pm, makes going to sleep much more difficult. One doc I heard said, "Don't let the train leave the station without you." He said that, if you paid attention, you'd notice that you might yawn at say 9 PM. This was your signal to be in bed. I've rolled my bedtime back to around 9:30 from 11 where it was for years.
Sometimes meditating myself to sleep works. If I'm stressed, I'll do stress reduction before meditation. This includes 10-15 minutes of yoga to get rid of the physical tension - 4 rounds of sun salute, then a few other asanas. I'll follow this with progressive relaxation & alternate nostril breathing, then meditation. I don't always do all of the above, many times just the yoga then meditation.
As to breathing exercises, you might follow some of these links: https://www.google.com/search?q=sleep+breathing+trick&ie=utf-8&oe=utf-8
There is one from this book http://www.amazon.com/Natural-Sleep-How-Your-Share/dp/0878572163/ref=sr_1_13?s=books&ie=UTF8&qid=1428674302&sr=1-13&keywords=natural+sleep
(I have the book, but can't find it in my post-move disorganization) that was trying to increase your CO2 level. You'd exhale completely, hold for 20??? seconds and repeat three times. Then breath in through nose out through mouth slowly while looking up at the "third eye." I may have the details wrong and will post it when I find my book. This was effective for me. A few moments and I'd be gone. Here is Google book with pages on CO2 & sleep: https://books.google.com/books?id=VcMelPjTqGgC&pg=PA149&lpg=PA149&dq=natural+sleep+carbon+dioxide&source=bl&ots=imKSZnTA8w&sig=auuJEmqeEzjsOkAUfpsiYMkvzP4&hl=en&sa=X&ei=Wt0nVfBFi7GiBMPCgHA&ved=0CCQQ6AEwAQ#v=onepage&q=natural%20sleep%20carbon%20dioxide&f=false
Book is here: http://www.amazon.com/The-Power-Breathing-Self-help-Spiritual-ebook/dp/B0049U489G
When I used potato starch as a resistive starch experiment, my sleep improved, for a few months... Si it is like the neurotransmitters became balanced and sleep was really good. Then they got out of balance the other way. It is something I plan to try to experiment with in the future with other resistive starches and quantities.
Now, I try to make sure I'm in bed pretty early and plan to spend 7-8 hours with my lids closed whether or not I sleep that whole time. Sometimes I'll wake up early, get up and do the yoga/meditation thing, then fall back to sleep during meditation.