Liquid Plumber gel helps, but it may eat your pipes. I was renting an apartment and I had to put duct tape over the overflow hole in the bathroom sink. The landlady finally got a plumber in there to snake/root the drains, and I replaced the exposed P-trap myself when he was gone. No more bugs until the bottom of the sink rotted out. Sink was replaced 2 months later (yeah, priorities) and I moved shortly after.
__________________ "Aw! Why does everything that only happens to stupid people happens to me?" - Homer J. Simpson