What is your phone model? The way the internal memory is partitioned varies between brands and models, so what frees up internal memory for apps on one phone will probably not work on another phone.
I assume you already tried the usual measures such as removing apps you no longer use, otherwise that’s a good starting point.
The next step is to go through your apps and take note of any preinstalled Apps you don’t use, such as free trial games, etc. that cannot be uninstalled. For example, my last HTC phone had Facebook, Twitter and 7Digital, which I never use.
Go into your app settings, which is usually in the Settings menu -> Apps, then scroll right until you are in the ‘All’ apps listing.
Find the first app you don’t use and go into it. Touch the following buttons and ‘OK’ each: ‘Force stop’, ‘Disable’, ‘Uninstall updates’, ‘Clear data’, ‘Clear cache’. Then repeat this process for the rest.
On my last HTC phone, I freed up around 500MB simply disabling and uninstalling the preinstalled app updates and clearing their data.
This method is also a handy way of freeing up RAM, as some preinstalled apps such as Facebook run in the background even if not being used, let alone signed into.