Bonus points if it's by a margin greater than 5%.
Success would probably rest on a better coalition agreement for the Lib Dems. If they were able to prevent the tuition fees debacle by getting the Tories to agree to at least prevent them rising, there would be less bad will toward Nick Clegg, which is what the referendum became about. Vince Cable said he thought they should have got the Tories in government to promise not to campaign against it, and that Labour should have been more involved, in OTL many were critical of the referendum because of the accompanying boundary changes which would be favourable to the Tories. If the Lib Dems got the Tories to abandon or at least water down those plans, maybe you would have seen more enthusiastic cross party support.
Those changes, or a combination of some of them, would give you a situation where the referendum is less about a still disliked (but not as much as in OTL) Lib Dem party, and more about a cross party effort with only a few Tories without particulary high profiles opposing it. Looking at the polls, a margin of victory by five or ten points was entirely possible at the outset of the campaign, even some months after the coalition agreement. If you eliminate the problems that the AV campaign had, it could well achieve that margin of victory. The only issue is that the Tories might not be willing to budge on so many changes. Are we allowed pre election PoDs? Perhaps having 'Cleggmania' remain right up until polling day by having the debate closer to then would give the Lib Dems more seats, and therefore a stronger position in negotations with the Tories.
Not that AV would have made much of a difference thus far. The result in 2015 would have been more or less the same, the Tories would have gained 7 seats over OTL, Labour down 5, the Lib Dems up 1, the SNP would have two less seats, and UKIP and the Greens would have remained on one a piece. Though of course a PoD where there is less resentment toward Clegg could see the Lib Dems do better than that in 2015.