how to make usb flash drive read-only?

[Raymond]

That tool doesn't make the USB flash drive read-only.
It only works when the software is installed on the computer.
When you plug in the USB flash drive on a computer that doesn't have that software installed, the USB flash drive will still be writable.

[parahumanoid]

While I'd like to have a "read-only" bit somewhere in the volume properties that I could set, we, apparently, live in an imperfect world, where no such bit is available (that I know of) and NTFS doesn't support read-only operation properly anyway, while FAT32 cannot store files over 2GB.

So, in a Windows world I settled with NTFS after all. So here's my take on the problem:
1. Set root folder readable to everyone, writable only to your home pc user account and make sure to propagate these permissions to all subfolders.
2. Create a folder "Inbox" (optional) and set write access to everyone. That is, if you want to have a writable folder at all. I believe having one is OK. Most viruses tend to nest in the root directory because they need the autorun functionality to reproduce, so as long as there is nothing dangerous stored directly in your root directory, your USB drive is safe to automount and you can always run a virus check on your inbox directory before opening it in explorer.

Better yet, refrain from using explorer as a shell (at least, as a file browser): too many things can be made to happen automatically there (can you say "malicious scripting"?). The dumber your shell is, the safer it is. TotalCommander is a good thing to try. If using explorer, use the Table view everywhere: faster and safer than executing the thumbnails.