Building a bankroll in online slots can be influenced by choosing the right games to play from the start. It's important to select games that offer a good balance of bonuses and payouts. One strategy is to start with a consistent bet size for the first 20 to 30 spins, and if no bonus or big win occurs, then consider doubling the bet until a significant win or bonus is achieved. Playing responsibly and selecting games that have a history of paying out well is also crucial.