How do you prevent your lasagne from being watery?

Moisture comes from several source. Meat, cheese, sauce, noodles (if preboiled). There a variety of things you can do to address each of these. As mentioned in my answer to your previous question, letting the lasagna rest prior to service will help. So too, using dry noodles, without boiling, will cause the noodles to absorb some of the liquid, leaving you with a still firmer result.

Lasagna is a dish with such a wide variety of recipes that there is no 'one' answer to resolve your issue. But, in addition to the input from your other recent lasagna questions I would add these points to address, in general terms, your question:

• To deal with 'water'(or myoglobin) from the meats, brown the meat separately and thoroughly so as to 'boil off' these liquids.
• If your meats are particularly 'oily' (say 70/30 ground beef or sausage) you can add some flour (2-3 tablespoons) after the water is boiled off and continue to cook the browned meat to for at least two more minutes while stirring the meat. This will form a roux on the meat that will both absorb the oils and then thicken the sauce as it cooks into it. Then build your sauce on top of your meats. Once your roux is formed begin to add the sauce elements to the meat. First add any broth, wine (or beer) to the meat pot to deglaze the pot and get up those wonderful bits of flavor that have caramelized to the bottom of the pan. After that add your tomatoes followed by the seasonings. (note: do NOT rinse/strain your meats, this will carry away far too much flavor)
• For moisture coming from the sauce (tomatoes, broth, wine...)allow your sauce to simmer longer before adding it to the lasagna dish. Again this 'reduces' the sauce and will leave you with less liquid at the end.
• Cheeses, choose well aged cheeses over some of the cheaper alternatives. (Ricotta over cottage, real mozzarella over that pregrated stuff in a bag, etc.) these cheeses are 'relatively' drier and will leave you with less liquid at the end.
• On more thing... if you still want to tighten up your lasagna some more, add a quarter to a half cup of panko to the sauce just before you begin to construct the lasagna. This will help absorb some of that moisture as well.

(a word of caution: your milage may vary...use these techniques sparingly to reach your desired outcome. If you over do it, you will end up with a 'too dry' dish)

When I bake a lasagna, I begin with the pan covered with foil. 350 - 375F for 45 minutes. Then remove the foil. Bake another 15 -25 minutes so that the surface browns and edges crisp. Remove from the oven. Rest for 30 - 45 minutes (this is critical for your specific question). Portion and serve. If you still find the end product runny, reduce the amount of condiment in the lasagna.

If the sauce is to wet. I add some dry fine ground cheese to absorb the extra water in the sauce. Cheaper sauces seem to be more water. Also dry spice added will absorb some.