The problem with floor drains is that they remain stagnant for such long periods that the trap water evaporates. The best solution is to fill the trap with a liquid that does not evaporate as easily as H20. Mineral oil (which is odorless) is a good choice, or you can use an aromatic oil with a low vapor pressure (i.e. that does not evaporate easily). Or you can just poor water down the drain on a regular basis, say every couple of weeks. Of course, I assume that your floor drain DOES have a p-trap (some are improperly installed without traps), and I also assume that it is properly vented (some aren’t).