Which is an effective way to resist peer pressure to do something you're uncomfortable doing?

The correct answer is - assertively and confidently say that you don't want to. If you let your peers know that you really don't want to do it, and that you're quite sure about that, then they will likely understand and won't push you to do something you don't want to do. If you show any hesitation or doubt, they might understand that as if you were thinking about it - so just be strong and confident when you reject them.