Avoid the August Coup. Do that and nine out of fifteen SSRs will likely ratify Gorbachev's New Union Treaty. It wouldn't be centralized, but it'd be much more of a super state than Yeltsin's Russia, avoiding much of the perceived humiliation on the Russian side from that era, resulting in a less nationalistic, aggressive foreign policy down the road.
Alternatively, there's the uglier route of some kind of coup against Gorbachev succeeding and putting people in charge willing to use military force to keep as much of the USSR together as possible. Even in 1991 the communist party was strong in the Central Asian SSRs. Byelorussia, in the meantime, didn't have a strongly developed national identity and could have been kept too (it's a Russian puppet now IMHO). Azerbaijan was also fairly pro-Moscow IIRC. The ugliest would have been eastern Ukraine, which would have needed to be attached to Russia manu militari, as mentioned at the start of this paragraph. The Baltics would have been ugly too, but could have been hung on to by virtue of Russia's sheer size. Heck, the 1991 coup might have done this if it had been carried out competently. This course of keeping the USSR together would require a lot of luck though. It could just as easily turn into Yugoslavia on steroids.