Our refusal to accept a referendum on membership of the EU is as poor as the Liberal Democrat's refusal to accept a referendum on Lisbon. We should offer a simultaneous referendum on both subjects, campaigning for a "yes" vote to stay in the EU, and a "no" vote on Lisbon. At a stroke, this would go a long way to eliminating the UKIP vote at the next election. It would also highlight that we are not anti-EU which campaigning purely on Lisbon would suggest.
This would not divide the Tory party. It could be presented as a free vote to all but the Conservative front bench. It would cross party lines just as other European votes have done. Announce it, announce it now. Do it quickly after the next election to heal any potential rifts it may cause. It is the single biggest thing my party could do to ensure victory at the next election.