There is only one solid method that ensures people will buy your game if they like it... look at Galactic Civilizations. When you buy the game, you get a key that you can create an account with - or something like that - which enables you to download patches and other bonus content. You can also play online (well, GC has this Metaverse thingy).
For Construct games, well, you could use unique keys to 'unlock' the content and extra features. When someone buys your game, you send them an unique key! Accounts can only be created with an unique key, so cloned keys can't be used to create a new account, thus one can't just distribute the key to 'friends'. It will work to unlock the game, but account-related features - patches, bonus content etc. - are only available to original accounts.
That is how it works the best in my opinion. Remote computers work as well; you just install the content you downloaded with your account, as long as the used keys match.