Anarchism

Some wire shelves to keep important stuff off of floor level. A shop vac for smaller flooded areas, or an extra sump pump if you have an underground basement.

I think some companies make these inflatable tubes that go around your house and you can fill them with water and they provide a barrier about 2-3 feet high.

It's probably expensive and uses a lot of water, and requires a clear space around your house, but it's worth considering if it's an option for you.

For a more permanent investment, you could dig a trench and mound the dirt up beside it, but this depends on what's buried nearby. You could also try routing it to an artificial pond of some sort.

Make sure the drainage ditches and culverts near your house are clear. If they're overgrown or clogged, either clear them yourself or tell your county to clear them.

Beyond that, sandbags can help with the gaps under doors if you're just dealing with a few inches.

You could talk to your neighbours, prepare an emergency plan for potential disasters

The best preparation for disaster is building community. Form tenant unions and labor unions. Join or form community gardens. Join your local library. Talk to your neighbors. Anything that creates cohesion with the people around you. Then, when a disaster hits, the community doesn't need to come together because it's already together

Sort of depends what your resources are, there's a continuum between doing nothing and full doomsday prepper. It also depends on what sort of flood you're preparing for. Like, 6 inches of water in the house, or the house is gone? Anything that gets touched by flood water is basically coated in literal shit, so you have to prepare for everything from the water level down to be completely wrecked but (hopefully) not utterly smashed like in a tornado.

Probably wise to keep some water and food. A small power bank for electronics. If you have expensive or irreplaceable things put them on an upper floor or at least off the ground if you can. If you've got some more money you could think about a generator, not just for you but for your sump pump (if you've got one). If you plan to leave maybe scope out a place you can go so you don't end up in a shelter. Figure out a plan for your animals, if you have any.

If you have insurance, take pictures or record everything you have. You want make and model of everything when it comes time to get reimbursed. You will forget a bunch of shit, and if you specify "coffee maker" you will get a $15 coffee maker, instead of the "Miele UltraCoffee 9000, Stainless Steel, with Grinder" that you lost (for example).

Have you all your important documents, photos in the cloud?

https://www.cdc.gov/floods/safety/index.html