Help Best way to uninstall all app data ??

What is the best way to uninstall an app from our phone that would also removed the databases or other applications parts it downloads? ive noticed some apps are really small bc it's just the app, then when u run it, it needs to download the rest of the program, like a dictionary database for example ... i dont care if the apps used the SD card cuz that space is practically free, lol but its annoying when it app uses the onboard memory which is limited to begin with ... so how do u make sure the extra data isnt still lingering after u uninstall the app?