3 ideas
Investors as a group earn the market return minus the costs they pay, so every dollar spent on expense ratios, loads, trading and advice comes directly out of the return. Because future fund performance cannot be forecast but costs are known in advance, choosing the lowest-cost fund is the one selection method that dependably beats the average.
Set your bond percentage roughly equal to your age and hold the remainder in a diversified stock index fund, then rebalance back to target on a schedule or when drift exceeds a set band. Rebalancing forces you to sell what has risen and buy what has fallen, which controls risk through a fixed rule instead of through market forecasts.
Investors typically earn less than the funds they own because they buy after strong performance and sell after declines, which puts their money in at high prices and takes it out at low ones. Choosing a sensible plan, automating contributions and deliberately doing nothing during panics captures returns that frequent tinkering gives away.