When I cook lasagne most of the filling slides out between the pages. How do you make the filling stay in its place?
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.
How exactly are you constructing your lasagna? If you stack it like this: noodles, red sauce, ricotta cheese, mozzarella, noodles, then it is going to make a mess. I construct my lasagna in an alternating pattern: noodles, red sauce, noodles, ricotta cheese, mozzarella, noodles, red sauce... and so on. And be conservative with the ricotta layer. If you're putting an inch of ricotta between the noodles, it's going to be difficult to keep it together. One last suggestion: make the red sauce as thick as possible. If you're using watery sauce, it will want to fall apart. And I agree with Cos Callis - time is your friend.