Well, after booking the flights and the guesthousing in Leuven the most important thing is planning everything you have to do before leave so as to be succesfully.
It's important to read carefully the dossier Living in Leuven from the KU of Leuven and has got under control everything related with the health insurance, passport, Visa... In my case, I had to ask for the European Health Insurance Card (EHIC), in Spain you can do it trought internet and they send the card at home (it's so easy and useful!). When you arrive in Leuven, the City Hall will ask you about the health insurance and you can show this card.
Another important thing is the International Student Card http://www.isic.org/ which will allow you to have lots of advantages. I haven't got it yet but I think it would be possible to get it here in Leuven. I'll ask for it!
About the luggage... oh it's a bit difficult to set up everything well! I only had to think about the first semestre, so my cases were plenty of sweaters, scarves, winter hats... I think the winter will be cold this year. Dispite this, I took summer and autumn clothes too. Don't forget lots of socks and underwear! I cannot give much advice about it, because it's a personal decision, but it can be useful to take a little iron, a sewing kit and something to clean shoes.
I took two big cases with clothes and shoes and another little case which was on the plane during the flight. I put there a clothes change in case they lost the luggage, the camera, the videocamera and other electronic things. Don't be afraid if the luggage is so heavy, it's normal :) so depending on your flying company you should pay more or not. Maybe you can plan with your family that they can send you most heavy things like books
I think I've said all about the luggage :) see you!