If you mean the internal storage, moving media to the external sd card, clearing caches (especially browser apps and social networking stuff), and uninstalling unused apps are things that will free up space.
If you mean RAM (which is what the word "memory" would imply on a computer), then don't bother. Android isn't Windows, and there's no point in keeping a lot of free space in RAM - unused RAM isn't doing you any good, so it's more efficient to let the operating system use it.