If it is something that will put the cop or department in a bad light, they will not help. But your best bet is to just ask.
How do I get a police officer to help me with a prank?
You don't. They have better things to do with their lives then to play games.
That's a good way to get arrested for criminal mischief.
You simply ask, if your polite about it, the worse they can do is say no. Although if they say yes they could lose their job and all their credibility (meaning they can never be a cop again) best bet would be to just go buy a cop costume
like this video where a cop is involved
Play a little B Ball with neighborhood kids, help an older person not enough cops spend the time to show we care. Acting the fool and prancing someone Not part of the job! Don’t you thing we look bad enough without adding fuel to the fire!