How to prevent the lasagne filling from sliding out?
Time...lasagna needs to allowed to rest for a while before serving. At least an hour. If you try to serve it straight out of the oven it will slide all over on you. Time will allow the cheeses and other filling to firm a bit to give you the distinct 'layers' that you want to see out of a traditional lasagna. I would even recommend making your lasagna the day before you plan to serve it and chilling it over night and reheating just before service.