Nylon washer and lamp wick (ball string cotton) behind the washer.

Some folks use Teflon on washer BUT it looks sloppy and doesn't compress as well

In general I when I have a leak it is because I was not careful about getting everything aligned before tightening.

However, the only joint that is constantly under water is the one at the "end" of the trap and that uses a flare and socket molded into the plastic ends.

Inspect those surfaces carefully. I had one with a bit of flashing from the molding that was about as thick as toilet paper and about 1/8" dia and it kept leaking until I found that.

Forgot to mention these are metal pipes and rubber washers....

Do these suggestions apply to these materials?

The flat rubber washers almost always leak.

You are much better off used the beveled nylon type and if possibly lamp wich NOT quick wick behind the washer HAND tight should be enough to make it leak proof.

One more point I never use plastic SJ waste only rough brass or CP brass and I order brass nuts not the standard white metal crap.

For about $2.00 per nut it is well worth knowing I have peace of mind and never less then 17 Ga for the fittings