Normally, with hard-top cars, the easiest way to clean the underside of the car is to run it through an automatic car wash. On convertibles, where doing this can cause damage to the top seals and subsequently leaks, how can you clean the underside, and are there any options to help it stay clean and rust-free? My first thought for cleaning is a power washer, but what are other options for when one isn't accessible?
Find a pressure wand that attaches to a garden hose (would be nice if it has a soap attachment). Even if the water pressure isn't as high as a pressure washer, it will be better than nothing.
You can put the car on jack stands to give you more room to work, if needed. Spray soapy water on the underside and let it sit for a couple minutes, then rinse.
I have also seen small dollies with spray nozzles attached that can be rolled back and forth under the car. Not sure where to get one, but seems easy enough to make.