Cartels by nature are extremely fragile and sensitive to conflicts of interest. The members of OPEC cannot abide by their strict quotas and that hurts every member of OPEC. If OPEC were serious about reducing the supply of oil (and hence raise their revenues) it should solicit deposits from its members to be paid back each quarter with (a natural rate of) interest if they abide by their agreed-to quotas. When each country knows that every other country has the incentive to keep their own production in line, each country will have additional incentive to keep its own production in check. A country that does not control its level of production will not get back its deposit.
Since every oil exporter will reap benefits from this, they will be able to afford those interest payments to ensure loyalty and to attract new members. If a country recognizes that not only can it raise the price of oil but also receive interest payments (for good behavior) by participating in the group effort, it will be more likely to join.
Of course, this is what I believe OPEC should do to ensure loyalty if I were it. But I certainly want them to keep back stabbing each other so we get cheaper oil.