Tag your users, and tag them good.
Make the sign-up for trials generate a simple link with a UUID or an actual high-entropy token to identify the source. That token should then attach itself as pervasively as possible. URLs, cookies, LocalStorage, you name it, that UUID should be there. It should be easier to click your magic link than to create a new signup.
You may find that you need to completely separate your trial authentication/authorization from real, credentialed users (i.e., Charlie doesn't get inside the chocolate factory without a Golden Ticket—the magic tracking token should be all that's required to try your software). In the spirit of the above suggestions, you will want all browser bookmarks created by your trial users reference the radio collar you crafted for them.
In general, the solution is more Economics than Software Design:
- Make your product easy to try.
- Make your product easy to buy.
- Make your product easier to buy than to rip-off.
Getting a trial should be as as you can make it—assuming you can actually convert your trials into real customers, you should be handing out free trials Wherever Web Apps Are Sold(TM).
A happy user of your trial product should find it easier to enter their credit card number than to figure out how to clear their cache, lose all their cookies and wipe local storage. Only people who can see the value in your product will be wanting to get more. Those people are your future customers.
Finally, spend your development effort to make it super-easy to buy your product. Engaging in an arms race with people who will never buy your product isn't worth the effort. Finding all the domains
mailinator.com has registered is far from trivial, and that's just one way an active user of your product can find unlimited email addresses.
Some final suggestions:
- Make sure your Take My Money link is prominent.
- Only require the bare minimum details to make the transaction go through. A fast checkout lets the money get to you quicker.
- Sign-ups should be plentiful and refunds painless.
- Reduce the value in creating lots of signups. If you have a way to restrict all free trials to a limited (not crippled, but limited) experience, then the value of a proper signup becomes even more compelling. For instance, if paying customers can create unlimited widgets, free trials can only create five. Enough to try, but too limiting for real use. If you have twenty chapters of something, try rotating the unlocked chapters every week.
- If you don't want to cripple the experience, keep in mind that you're allowed to make them think that the trial is still working after the trial is up. Maybe they find a sudden, unexplained increase in the amount of promotional banners on all the pages.
Also, if "many of [your] users are over 60" (sic), you may want to make sure that any of your proposed anti-hacking efforts don't become obstacles to likely sales.