I know for two reasons.
One - i often restart games for more 1-time items and pokemon (Like Palkia and TM-Grass knot)
Two - i first found out by looking at the instruction manual
I thank that one would be the best way to find out lol. Then I know most of us don't care to read the insrtuction manual we just want to jump into the game but yeah that's how it is done and I thank the game tells you that you have to do that when you push the new game oppsion in the main screen. :wink: