Check out Lifesharers.org, a group that's trying to encourage people to donate organs at their death.
The incentive to do so? Priority to receive organs from other members, if you need them, without being stricken from other sources (national waiting lists, etc.)
It's completely free to join. Joining LifeSharers is what gamblers call a freeroll--a chance to come out ahead, without any chance of coming out behind. The worst thing that can happen is that every existing organ donor will join LifeSharers -- at which point lots and lots of people who have already agreed to donate their own organs will receive priority in receiving organs. One of the best things for society that could happen is if everyone joins LifeSharers -- at which point the pool of organ donors will swell tremendously. As an organ donor, I feel that it's simply a smart thing to do.
Note that LifeSharers is completely free, and you can "unjoin" at any time.
If you need more persuading, you'll need to get it from someone else. If you want to argue about whether it's right or wrong to receive an organ transplant that you need, you'll need to argue somewhere else. And if you want to receive an organ from me if you're not an organ donor yourself? As far as I'm concerned, you'll need to get it from someone else.