Secret Bonus Round Room
To make this trick work effectively, you must use saved games. On your last life, you must die playing as Diddy Kong. At the Game Over screen, press Start. When Cranky Kong appears on the screen, press Down, Y, Down, Down, Y (DYDDY, as in "Diddy").
Immediately a room will appear with three gold Rambis, three gold Winkys, three gold Enguardes, and three gold Expressos. You may enter any of these four bonus rounds as often as you like, to collect as many 1-ups as you want.
To leave the special bonus area, press Start to pause, then press Select. You return with all your new 1-ups to the area where you last died.