Myth: Pressing B and Up will greatly improve the chance of capture when throwing any type of ball.
An oldie but a goodie. Many rumours of various button combinations have popped up throughout the life of the Pokémon games, all of which are false. The factors that affect the chance of capturing a Pokémon are it's current HP compared to it's total HP, a status effect it is under (Asleep and Frozen being more effective than Paralyzed and Burned), and it's predetermined Capture Rate - the lower this is the lower chance you have of capturing it.