Raptor migration is a beautiful and amazing thing to behold, but indeed it can be very stressful for us chicken keepers!
Possibly the most foolproof solution would be to keep the birds secure throughout the migration period... safely in a fenced run with a roof/netting on top, if you are able.
If this is out of the question, now is a great time to start tucking away a little money here and there to build a secure run in the future (maybe next year?) for times when your yard is not safe! Trust me, this comes in handy in other situations too.
Until then, you can try a few things. Try free ranging only when you are able to physically be in the yard with them, and easily visible to any hunting raptor. This is still a bit risky as some people have had bold (or desperate) hawks strike chickens even when they are nearby.
Likewise, if you have a dog that is safe around the chickens, and is easily visible to a hawk, this can be a deterrent to a raptor as well. If you are keeping adult large fowl, any raptor other than eagles or great horned owls will need to take their meal on the ground, and experienced raptors will not do this in the presence of a large ground predator (humans/large dogs) because it's too risky. But, sometimes young inexperienced birds will still try their luck (this is one reason why the mortality rate on first year raptors is very high... sometimes more than 60% die in the first year).
If you don't have a trustworthy dog or the ability to be out with the chickens, you can try a scarecrow and move it daily where the chickens frequently go. All the better if the scarecrow can move a little in the breeze. This is hardly foolproof though!! Many hawks are not deterred. This should only be considered if you don't have any alternatives, and you must realize that it might not work.
All of the other cheap suggestions that are commonly thrown out there are generally pretty ineffective. Some people swear by fishing line and CDs, but as you and many others have experienced, it hardly works all of the time. Owl decoys rarely work against hawks and may actually attract them (many raptors like to harass and drive away owls). Spinners, pinwheels, radios playing, noise, etc... are generally not that scary to a migratory hawk, especially if they are an older bird that has seen it all before in dozens and dozens of yards along their migration route...