Have Valery Mironov become much more hostile towards separatism within the USSR and arrange someone else replace Dudayev as a commander of 132nd Heavy Bomber Aviation Regiment in Tartu by the time Moscow orders an early full crackdown of Baltic nationalist movements.
Instead of the limited violence that occurred in OTL, the three republics are thrown into full chaos as embittered nationalists start a terror campaign against the local Russian-speaking population. As the local Russian communities demand protection and situation deteriorates further, this creates a fatal dilemma for Yeltsin - he simply cannot credibly maintain his image as a Russian nationalist and abandon a sizeable Russian minority in the Baltics.
http://en.wikipedia.org/wiki/Dzhokhar_Dudayev#cite_note-Uzel-0