Ralph to some extent does impose his power over Jack early in the novel. When Jack lets the fire go out (Chapter 4), Ralph repeatedly insists that Jack is at fault until Jack reluctantly apologizes. When Jack hits Piggy breaking his glasses, Ralph again repeats, "That was a dirty trick." Quietly, nonviolently, Ralph puts Jack in his place.
As chief, Ralph is the one to make the rules. This responsibility is not one that he takes seriously until Chapter 6 after he realizes that Jack and his group have let the fire go out. Ralph establishes fundamental rules that the boys need to follow in order to survive and to be rescued.
However, what Ralph cannot control is the fear on the island. In Chapter 6, when he opens the meeting up to the discussion of the beastie, the meeting breaks up into chaos with Ralph fearing that the boys will no longer heed the sound of the conch. What Jack instinctively knows is that the one who can control fear is the one who gains power. Because Jack can manipulate the boys' fears by leaving a sacrifice for the beastie, killing Simon who supposedly was the beastie, telling the boys that the beastie comes in many forms, Jack becomes the ruler, and Ralph's power which relied on the goodness of man is diminished.
What could Ralph have done? I don't really know. The boys were children, innocent, scared, and easily led. He tried to appeal to their more civilized natures, but their fears and desire for fun took over, so they turned to Jack. We see this same type of thing happening today. Ignorant and fearful people are particularly vulnerable to a demigogue who promises protection against an imagined threat. Ralph needed a more mature, a more enlightened group to lead, to render Jack powerless.
I do not believe that Ralph could have done anything any different with the exception of being more violent and vicious than Roger and Jack. Ralph has demonstrated his ability as a good leader. In the beginning of the story he collects the boys and makes decisions on the behalf of the collective group. However, the basic need for survival becomes apparent and that is that before housing or rescue comes the need to eat. Jack holds the power in his drive and ability to kill.
Perhaps had Ralph been more talented at scoring meat for the group, the others might have continued to look towards him as a leader, but Jack and Roger made a pretty menacing team. Once they had lured the boys with food, they used bullying strategies to keep them where they wanted them.
Ralph stands up to Jack rapidly in the novel and displays that he is not afraid of Jack. They even come to confrontation towards the end of the book, but Jack has more people on board and has no social limitations to prevent him from actually following through and executing Ralph.
Ralph's nature will not allow him to succumb to the display's of violence and cruelty that lies in Jack's personality.