Wash in a fine-mesh strainer, massaging the seeds with my fingers until as much loose goop as possible is gone. Then transfer to a bowl and soak for 30 minutes with a tiny bit of Oxyclean. Then rinse in the strainer again. Spread on a plate or tray to dry.
The Oxyclean breaks down gels much like fermenting would, but it's faster and less smelly. It also has the added bonus of sanitizing the outside of the seed, so if you have problems with plant diseases this stops them from transferring.
I should mention, I'm usually working with large quantities, so this may not always be practical. With very small quantities, smearing them on a paper towel and letting them dry works fine. Rip the paper towel up and plant it with the seeds still stuck.
I ferment mine. My only messy ones are cantaloupe and tomatoes and in a kid sized glass you can ferment hundreds of tomatoes and dozens of squash. Put it in a closet and let 'er rip for 4 days, wash and pour off the floaters and junk, strain, dry and store.
"Hundreds of years from now it will not matter what my bank account was, the sort of house I lived in or the type of car I drove... But the world may be different because I did something so bafflingly crazy that it becomes a tourist destination"
Location: Zone 7b/8a Temperate Humid Subtropical, Eastern NC, US