Be sure you're using enough cornstarch, as this thickens the fruit juices while the crumble bakes. If using frozen strawberries or rhubarb, thaw before baking in order to blot away as much excess moisture as possible. If your fruit is still looking extra juicy, consider increasing the amount of cornstarch used by a teaspoon or two. Lastly, let the strawberry rhubarb crumble sit for at least 20 minutes before scooping as the juices will solidify more as it cools.