Support universal healthcare
in the US, and in other countries do everything you can to fight the trend toward privatized healthcare.
One major contributing factor to homelessness is physical or mental disability or mental illness, and my favorite example of most homeless people created with one law is from my home state of North Carolina. See, there used to be this state program that would buy anti-psychotic medication for patients who couldn't afford it and didn't have other health insurance. That program was cut a few years ago. Before it was cut the psychiatric clinic where my roommate worked served about thirty patients who got their medication this way. They had jobs, apartments, and something approaching a normal life, but at little more than minimum wage and no benefits they couldn't afford their medication, so it was provided for them. Once it wasn't anymore, many of them just dropped out of sight. Some were picked back up by other programs or family members who helped out, but quite a number are now whereabouts unknown, most likely homeless since it would be very hard for these people to hold down jobs or permanent residences with all the voices screaming in their heads. The effects had to have been even worse at clinics in rural or economically depressed areas.