Having some time on my hands, I've just been "free-ranging" around the message board and have noticed that people are having a lot of trouble regarding their luggage - from deciding what it should be, to deciding what to put in it, to having it stolen, to having it lost by an airline (ANY airline)!!
I realize that this simply isn't an option for everyone but I've learned (the hard way...)that traveling is ever so much easier if you do it with a minimum of "stuff". If you can possibly get away with it, try traveling with only one bag, that you can manage by yourself under all circumstances (save illness or disability) and that contains nothing of great value. Keep important documents and money in a hotel safe or on your person. Leave the "serious" jewelry at home. It has been my experience that most Spaniards are far more impressed with a dignified and neat appearance than with a show of wealth.
Remember that Spain is a modern country now and almost anything you might need can be purchased there - should that need actually arise (I have a friend who packs an incredible number of things that she "might" need in almost any conceivable situation!)
I know there are several posts about traveling with only a backpack - by the way, those bags that convert from rollerbag to backpack are WONDERFUL - I'm just depressed that so many people could be sabotaging what should be an exciting and carefree vacation by taking along too much stuff!! Let's have more tips on how to avoid the luggage hassle from some of you who've done it.