To get unlimited lives just go to world 2-4 and near the end you will see a turtle walking down the steps. Jump down on him, but don't kick the shell. Next, get on the step below the turtle shell, push up against the step and then jump straight up without touching the D-pad. You;ll start to bounce on the shell and the 1Ups will start to just roll in.
This sounds vaguely familiar, but I think I've played more SMB in the last 2 weeks than in the last 10 years.