The best way to keep baklava from getting soggy is to avoid adding too much butter. You want light coatings of butter on the layers; otherwise, the butter will seep out and create a soggy bottom. You also want to make sure that you do not slice all the way through the baklava until after you have poured the syrup on. Slicing before could cause residual butter and syrup to pool in the bottom of the pan. If you've followed those steps and are still getting a soggy bottom, try letting the pastry fully cool before pouring hot syrup over the top.