Stalin was appointed the high position of General Secretary of the Communist Party which gave him the authority to appoint posts and control the membership of the Party. As Party Secretary, Stalin could ensure that he had his supporters at every level and could give them more power than the opposition. This gave him a link of power throughout the Party which gave him control and a solid chain of support.
Stalin used certain devices to build up his influence in the years before becoming supreme ruler. He joined forces with Zinoviev and Kamenev, old Bolsheviks on the Left Wing of the Party, who helped him defeat Trotsky, subsequently turned on them and defeated them by joining forces with highly intelligent politician Bukharin, and putting forward his ideas on Socialism in One Country which gained him wider support. Finally, Stalin turned on Bukharin and right wing MPs, attacking the NEP which they supported. He now had enough power to have them removed from their posts.
Stalin played cunning games in order to gain what he wanted. He played Communist Party groups off against each other, and in doing so, showed himself to the public in a better light. At Lenin’s funeral Stalin tricked ill Trotsky into not coming so people would assume he couldn’t be bothered to turn up. Stalin made a big speech praising Lenin and called himself Lenin’s disciple. This gained him popularity and trust.
Luckily for Stalin, Lenin’s testament warning against him was kept secret from the public because it contained criticisms of other leading communists as well. This was a great relief for Stalin and meant he could continue to rise to power.
In the five years following the death of Lenin, Stalin managed to gradually and subtly gain power and control over the country and its people. He manipulated politicians and the public into trusting him and used them to secure more authority for himself. Stalin knew precisely what he wanted and was determined to succeed and therefore his frame of mind had a great deal to do with his progress. Although overcoming Trotsky allowed Stalin to continue his rise to power safely, the fact that he had the position of Party Secretary meant he could control how the party worked to some extent and could easily build up his power by ensuring he had a strong link of supporters inside the Party. Without Stalin’s position of General Secretary for the Communist Party it would have been extremely difficult for him to win the power struggle between himself and other politicians and build up his power to such an extent that enabled him to assume complete control over Russia in 1929. I therefore think this was the biggest reason for his success.