Great thread. Here are mine:
(1) As tempting as internet-buying is, don't buy a bag unless you can see it IRL first. Size is deceptive in pics, and weight is virtually impossible to judge. If you do buy on the Internet and you haven't seen the bag in person, make sure it is returnable.
And, my big lesson learned: I always think my next bag will keep me satisfied for a while. Never happens. No matter how many bags I have, as soon as I get that "last" bag, my mind starts working on the next "must have". Maybe I'll get there someday, but I'm beginning to doubt it.