As far as food is concerned, I avoid purchasing ahead - mainly because of health considerations. However, I always have a packet or two of spaghetti, cornflakes, rice, etc.
One thing that I usually pay ahead is my Internet access. I usually pay for 3 months at once (the provider does not accept more) and then even if I don't have money, I know that I will be online.
Toilet paper needs a lot of space and I usually have no more than 10 rolls in stock. Soap, tooth paste, shampoos, detergents, medication, though generally durable, usually have an expiry date and I also don't think it is healthy to store them for long time.