How do I stop a drain from leaking from the tailpiece threads under a sink?

I just installed a new bathroom sink. The drain is leaking from under the sink, apparently from between the rubber washer and the threads on the tailpiece. The nut is tight. I read somewhere that sealant putty can be placed on the threads prior to tightening, but I'm not sure what sealant putty is. The tailpiece is plastic, so I'm assuming plumbers putty should not be used. Please tell me what sealant putty is appropriate for this or any other solutions. Thanks.