That's true for most commercial proprietary software, I think. It is probably the reason why games seem to be moving towards making money in other ways besides actually selling the software (or they introduce some horrible DRM that makes the game half-unplayable and encourages further piratingJussi Koskela wrote: the most common word of mouth seems to be how to play the full version illegally for free.
It's pretty descriptive if you check what additional keywords Google suggests when you search for 'Deluxe Ski Jump 4'.

Here's a list of totally crazy suggestions (based on what I have seen in other software...):
- start selling a mobile app for tracking player progress, watching replays etc.
- start selling customized jumper helmets, suits and skis
- make the game completely online: no offline-mode or compulsory connection to DSJ-online even when playing offline (with monthly fees)
- include advertisements and have users pay to get rid of them

Interestingly, as strange as it sounds, the main reason to pay for DSJ seems to be honor: getting your name on the top of that list, showing your skill online. Really dedicated players care about this and will keep paying for it no matter what, but if you just want to jump now and then offline with your friends, the treshold to piracy (probably) gets lower. I think you should find some other element to attract more players. For example, a lot of players seem to be interested in what I would call role-playing elements such as a career mode. While I don't think this is really what DSJ is about, maybe you should consider something like it. Currently the game is a pure skill game, I think current developments in gaming show (sadly, in my opinion) that people are more interested in role-playing, social features (customization, achievements, social media integration) and the casual easy-to-play-instant-reward type of games (preferably on mobile devices). All of that sounds awful to me but you need make money somehow. One thing I would like though, is multiplayer over LAN or possibility of hosting private games online.