Well the best lessons are the ones you learn the hard way.  I went on my most recent camping adventure with my tent and it started to rain.  Unfortunately, for my group I had not sealed or waterproofed my tent.  When it started to rain water started leaking in through the seams.  Additionally, I forgot to bring the other piece of the tent called the rain fly.  Later when I got back home I asked some of my buddies and they said that they always get a simple tube of sealant from the local sporting goods store.  They also said that they usually put the sealant on with two coats to make sure the seams are good and waterproof.  I guess I’ll know better next time.