How to free the unused occupied memory?

I had opened the task manager, watching how the memory usage of my program changes as I work with it - and it was only ever going up. I thought using Dispose() will free up the memory, but apparently it doesn't. Just a notice here, the dispose method I'm calling is of a custom made control made by someone else. Then I wrote GC.Collect(); after the code I close the form which won't be used anymore, but this had an insignifficant effect.
So how can I free up the unused memory? Thanks in advance.