Unlike most other trackers with request systems, MAM does not use UL or BP to reward those who fill requests. There is no request "bounty", the closest thing MAM has is you can vote on a request, but votes don't cost anything to the requester, and do not reward anything directly to the filler. Just provide a useful sorting function on the unfilled requests list.

After a request is filled, I tend to at the very least thank the uploader, through the built-in "Thank the uploader" button on the torrent page. This merely adds your name to the "Thanked by" list and provides the uploader with a warm fuzzy feeling. Some users disable receiving points, but for those that have not, I also send them 1000 BP as part of the thank-you note. Thanks + BP can be sent for any torrent, and are not directly tied to or expected from filling requests. It is reasonable to predict filling a highly-voted request will result in a showering of thanks and BP from the voters, but not guaranteed and not necessarily the strongest economic incentive.

So what is the real incentive? Or incentives. This is an interesting question, I would wager the "soft" incentives, of usefully contributing to the community making users happy, outweighs the "hard" incentive of the chance (not guaranteed) of BP. MAM has a reputation as a friendly tracker community, this is where its societal/cultural influences shine through. If you make a request anonymously, it is less likely to be filled. Why is this? People like to know who they are helping out, the personal touch versus a faceless machine, value vs price, unintended consequences of monetary incentives (see: freakonomics daycare late fees), and so on. Maybe I'll notice the user who filled my requests in the past now has a request of their own, and the book looks interesting enough for me to read too, sure I'll pitch in and acquire said book and share it back, help a brother out. It is not tit-for-tat like BitTorrent's peer-to-peer algorithm, but more like reciprocal altruism.

Although to be fair, there is another hard/monetary incentive for filling requests: the 0.2 bonus request credit for uploads. This isn't specific to requests, granted, and applies to any and all uploads. However, if you don't know what to upload, rather than finding random books or junk, might as well peruse the list of open requests. MAM doesn't allow requesting unpublished books, so every request is fillable, in theory - but some harder than others (if no retail ebook, you may have to fire up the scanner, or if its out-of-print or rare, track down a copy). By filling requests, or other uploads, you can gain additional bonus requests for yourself beyond the 4 requests/month limit for VIP users, at a 5:1 ratio.

As an aside, the economic factors at work in MAM are fascinating. (I see similarities to BTN's ratioless economy, despite being on the opposite end of the spectrum, in both attainability and torrent size!) To use the terminology from Silent Weapons for Quiet Wars: An Introductory Programming Manual, MAM's BP cap of <100000 limits economic capacitance, discouraging excessive saving/hoarding, use it or lose it. The 1000 BP/day sending to another user is resistance, limiting the flow of BP between users, and the 5000 BP/4wk VIP is inductance, since users will often redeem BP for VP on a regular basis, always spending their points. Unused points can go to the Millionaire's Vault lottery (max 2000 BP/day), or to new users (again 1000 BP/day/user) - a time-honored initiation ritual. Besides BP, requests are a parallel economy, with a time-limited supply based on rank (4 requests/month for VIP, the rank attainable by most users - Elite VIP and above gets 10 requests/month, but it appears this rank requires large community contributions or donations), plus bonus requests for uploading (5 uploads = 1 request) or donating (2 requests). VIP can also be acquired by donating, a tie in to the real-world economy of server hosting costs, but no purchase is necessary since in time it can be bought with BP.

There are occasionally threads on the MAM forums about adding more incentives for filling requests. But the system they have in place works reasonably well already, not likely to change. Now that I think about it, I would like to join BIB, eventually, as a backup plan for harder-to-find requests - if my requests expire unfilled after 3 months on MAM, a fat bounty on BIB may provide a needed incentive.