To get an extra 1-up, beat each stage without getting hit. If you come out unscathed (no, you cannot get hit and eat a frankfurter), you will recieve a 1-up after every stage you complete.
After collecting the Key item which goes into your sub-weapon box in Stage 3, if you have 15 hearts or more, use the ItemCrash button. While to most people it seems to do nothing more than make a question mark appear over your head, if the key itself makes contact with an enemy or boss during this brief animation, they'll sustain heavy damage. The best part of this secret is while you must have 15 hearts or more to enable the itemcrash, actually using it consumes ZERO hearts allowing the attack to be used continuously! It also has the highest attack power killing most enemies in a single hit even if used regularly as well (Up + Attack) and once again consumes no hearts however to balance it's attack power, it's also the closest range "weapon" in the game needing to be used immediately in front of the enemy.