The best way to prevent a soggy bottom is to avoid putting the filling on the pastry until right before baking.  The lemon juice and sugar will pull the moisture out from the fruit filling as it sits, which will produce liquid.  Bake the galette on a metal cookie sheet for the crispiest base, and make sure that it cools on a wire rack for proper air circulation. Another tip is to avoid using liquidy fruits in the filling, such as strawberries.  Tossing the fruit with a tablespoon or two of cornstarch or dusting a little semolina on the crust before adding the filling helps keep the pastry crispy.