In my opinion, the Bolsheviks probably wouldn't have come to power, or at least it would take them longer, if one of these two things happened:
a) WW1 wouldn't break out - or it would break out but Russia would remain neutral.
b) WW1 broke out, Tsar was overthrown, and Kerensky's Provisional Gov't would've called for immediate elections to the Constituent Assembly or would've pulled out of war and began land/social reform, economic reform, etc.
a) My reasoning behind the first is that the Russian Tsar still had support in 1914. By 1915, it began to drop and the Tsarist regime under his wife in 1916 lost most, if not all, of its support. Had he remained neutral, then Russia would still have the Tsarist Monarchy. Of course, had the Germans launched an invasion on Russian territoriees in Poland, Ukraine, and the Baltic states, then the population probably would have supported a Russian war effort - it would've been, after all, for the defense of their homeland. Probably the February Revolution would be reduced to just petitioning and a few isolated demonstrations that the Tsar create a democratic, directly-elected Duma, in effect transforming absolutism into constitutionalism.
b) You must remember that, since the Provisional Gov't refused to pull Russia out of the war or carry out land/social reform to appease the peasant and farmers until the Constituent Assembly convened on these matters, anarchy ruled in the rural areas - the peasants and farmers began to carry out land reform on their own through violence by seizing the properties from the nobility. This severely hurt the Provisional Gov't ability to function, as it did not have the support of the majority of the Russian people. The Provisional Gov't also refused to call elections to a Constituent Assembly - it constantly pushed back the date that the elections would be held, giving the Bolsheviks and other extremists more time to rally support against the Gov't and gain control of the Soviets. In the summer, a right-wing military uprising broke out. Kerensky claimed that it was a counterrevolutionary rising and moved to suppress it. However, he had by then begun to lose the favor and support of the left wing. Basically, while the PG was running Russia into ruin, paralysis, anarchy, and inflation, the Bolsheviks began to take over Soviets to "complete the socialist revolution and bring Russia out of constant disorder and paralysis".