Personally I really don't mind that the system is a bit leaky. I try my best to return all of my DBTCs back and dont really worry about them after that. Of course, your dbtc, your rules. Personally I love forum games for DBTCs. Also, you could theoretically track your DBTCs if you want. Just say when you give them out that they have to go on their sig, and tell you when they've given them away, and that they are inelligible for another one from you until they do so. But I like the system. Half my interaction with other reefers has been either picking up or giving away DBTCs. It's always fun to go see someone's tank and talk with them about it.
Edit: One possible suggestion might be to "strongly suggest" that people keep their signatures updated. Not only does it keep you honest publicly, but everyone forgets where they got everything. If I didn't use my signature, I'd probably forget what I owed too.
Edit2: Plus if I post that my tank crashes and I post asking for people to send my DBTCs back my way, people damn well better speak up!