There are several different ways to protect a building from flood damage. One way is to keep the water away by regrading your lot or building a small flood-wall or earthen berm. These methods work if your lot is large enough, if flooding is not too deep, and if your property is not in the floodway. Be careful not to simply divert or back up water onto your neighbor. Another approach is to make your walls waterproof and place watertight closures over the doorways. This method is not recommended for houses with basements or if water will get over two feet deep. A third approach is to raise the house above flood levels. A small wood frame house can be elevated for less than $10,000.
Many houses, even those not in the floodplain, have sewers that back up during heavy rains. A plug or standpipe can stop this if the water doesn't get more than one or two feet deep. They can be purchased at a hardware store for under $25. For deeper sewer backup flooding, talk to a plumber about overhead sewers or a backup valve.
These measures are called flood proofing or retrofitting. Important note: any alteration to your building requires a permit from the Building Department. Even regrading or filling in the floodplain requires a permit from the Development Services office.
If you know a flood is coming, you should shut off the gas and electricity and move valuable contents upstairs. It is unlikely that you will get much warning, so a detailed checklist prepared in advance would help ensure that you don't forget anything.