I would suggest you export by platform, then make sure the xml file under the Data / Platforms folder, the roms under Games / platform name, and the pics under Images / platform name are there and consistent.
So for example, if you exported a bunch of SNES roms, then decided to export another SNES rom later, you should export all of them together again. Replace the platform xml file, then add the new rom and images for the new rom.
The XML file from an export will list all the roms you've exported. Doing a single later and replacing the xml file on your android would then only show the single rom in launchbox.