How to get rid of sewer smell in an apartment?

If the smell comes back quickly after cleaning the trap, it's probably not the problem. Check if there's a clog deeper in the pipe - try to break it up with a plunger or call a plumber with a cable to clean out the main stack completely. It could also be that the trap has dried out - pour some water down the sink, leave it for an hour so the water seal is restored and smells don't come up from the stack. If the smell doesn't go away after that, you definitely need a professional, because the problem is either in the main stack or with a problem neighbor upstairs - you won't be able to figure that one out yourself.