Colin, because the stuff dries rather than cures every time you open a bottle you let air at it and over time it starts to thicken. I would decant a smaller amount from your large bottle and use it out of a small container and leave the big bottle alone until you need to fill the small one.
It's also worth remembering that you can thin PVA with water, which opens up a whole range of possibilities. I've actually put very delicate stuff together in the past and held it in place while I apply a spot of watered down PVA with a brush. I've used it as caulking mixed with black powder paint and I've laid deck plates down made of heavy paper with watered down PVA painted on.
I think it is one of the most versatile glues available.
