I've learnt to think of my pinhole images in terms of softness. It's like learning to see what will work in B&W. You can talk about pinholes and sharpness, it's all relative but not really worth the effort. I mean if you want sharp, use a lens. Pinhole excels at softness, you can have more or less softness but that is what you are working with. Accept the softness and you can start to consider the qualities of softness and what they offer. So the method described by DWThomas, sanding the pimple, gives a much sweeter quality than popping a hole into tinfoil. Actually, for sharpness you want to be using a laser. There are other variables to consider too, reciprocity is definitely a factor in pinhole but also there is a great difference between a paper negative and a film negative. It's all good. I think pinhole, zoneplates and the whole raft of lenseless photographic methods is a delight and I would only encourage you to do it.