3rd Gen FR/LG: How Do I Simply "End My Turn"?

I don't want to do anything during some of my turns. I don't want to attack, use items, or switch pokemon. How do I end my turn without wasting items or PP?

Are there any workarounds, or easy loopholes, if a solution doesn't exist?

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Also, do stat enhancing moves maintain after a pokemon switches out? If I raise evasion on blastoise, switch him out then back in, will his increased evasion remain? Will it return to it's normal level?

There's no easy solution, but you can use up two turns by switching out and then back to the same Pokemon.
And stat-enhancing moves only last until the end of the battle or until you switch out. When you switch back in, the effect will be gone. This also applies for some negative non-stat conditions, including confusion.