The thing about 1989 Beijing Protests was that it never had any unified goal or real leadership. Some were asking for a cleaner regime, some western style democracy, others returning to the old style communism. As per Zhao Ziyang, because the conservatives were so stiff and refused dialogue, any "leader" elected by the student were solely on their degree of radicalism, and could be replaced by someone more radical. The sentiment among the protesters was such that the majority(99.9%) felt obliged to follow the 0.01%, if one student stayed, the rest would not leave the Square (As student leader Uerkesh Davlet put it). This way, no government promise or concession could persuade the student to leave. It's either a crackdown, or the government had to be changed.
The other quintessential element would be the army. The Army had to act in favor of the protesters.
So, possible way out:
1) Some troops had to join the protesters. The protests worked only as a trigger to an armed revolution lead by the dissatisfied army, like in Romania. A much earlier PoD is required.
2) A much better organized protest. Its leader, like Gandhi in OTL, had to be able to call off the protest as he see fit, and any decision to withdraw (like in OTL's May 30, which was ignored by the student leaders) had to be faithfully followed. This way, the initiative would be on the hands of the protesters, not the government. The fear of provoking another protest would prevent the government from having large scale reprisals.
3) The crackdown took place much earlier, and was put down with less bloodshed. Deng had a stroke. The reformists saw this as a popular mandate for a coup, and after winning the army on their side, and put Li Peng under house arrest, forcing Deng to retire. I.e. the recreation of what happened after the other
Tiananmen Incident in 1976, which eventually lead to the removal of Gang of Four.