Entry in progress—B.P.
     
Google News Archive
2 February 1984, The Spokesman-Review (Spokane, WA), “Banquet attracts 900 stars” by John Blanchette, pg. 21, col. 4:
On any player who spikes the ball after a touchdown: “I don’t like them, either. I always tell the player, act like you’ve been there before.”
(Jim Tunney, National Football League referee—ed.)
   
Google News Archive
21 September 1984, Spartanburg (SC) Herald-Journal, “Notes and Quotes From Here & There” by Luther Gaillard, pg. C1, col. 1:
And though the Gophers have scored four touchdowns, Holtz says, “I don’t let my players do those crazy dances in the end zone because I want them to act like they’ve been there before.”
(University of Minnesota football coach Lou Holtz—ed.)
   
New York (NY) Times
SPORTS WORLD SPECIALS; Noteworthy Holtz
By Thomas Rogers
Published: June 16, 1986
Seven memorable things that Lou Holtz has already said since arriving at Notre Dame, even before coaching his first game there:

On why he doesn’t want his players dancing in the end zone: ‘‘We like to act like they’ve been there before.’’
   
6 January 1988, Miami (FL) Herald, “Canes No. 1, but aren’t tops in deportment,” pg. 1BR:
“Try to act, Mr. Trumpy, as though you’ve been there before,” Brown said. Mr. Trumpy did.
     
New York (NY) Times
Sports of The Times; At One Time, Paul Brown Jumped for Joy, Too
By IRA BERKOW
Published: August 7, 1991
(...)
Brown wasn’t wild about some of the new stuff in pro football, like dancing in the end zone after a touchdown. He told his Bengal players, “Act like you’ve been there before.”
 
Google Books
September 1991, Cincinnati Magazine, pg. 108:
“Son, act like you’ve been there before.”
—The late Pail Brown, chiding Bob Trumpy after Trumpy celebrated his first touchdown for the Bengals by tossing the ball into the stands.
 

47.Tom Blogical said:
posted on October 6, 2007 10:08 AM — 24.95.77.153 — link — abuse?
“One thing that you have to realize about most Canes fans is this. We are arrogant.”

We know. Trust me, it’s not something to be proud of. And it reminds me of a story.
 
Bob Trumpy, former ex-Bengal player and now broadcaster who used to play for the immortal Paul Brown, tells frequently of his touchdown catch that was the first in Bengals history. In his own words:
 
“I caught the first touchdown pass in Bengals history, and it was a nice catch. I was excited about it and celebrated. My parents were in the stands, my wife was in the stands and I was excited about the catch. I came off the field and there was Paul Brown. He brought me over and just told me, “Act like you’ve been there before.”
   
Google Books
Sportsmanship in Youth Athletics
By Tom Robinson
Edina, MN: ABDO Pub. Co.
2009
Pg. 43: 
Many football coaches have a simple opinion on how a player should act when he arrives in the end zone to score a touchdown.
 
Famous former coaches Darrell Royal (University of Texas), Vince Lombardi (Green Bay Packers), Paul Brown (Cleveland Browns), and Tom Landry (Dallas Cowboys) are among those who have been credited with saying, “Act like you’ve been there before.”
 
RedState
Democrats Need to Heed the Wisdom of Darrell K. Royal
Posted by Erick Erickson (Profile)
Monday, October 18th (2010) at 12:05PM EDT
It was Darrell K. Royal who originally said, “When you get to the end zone, act like you’ve been there before”
(...)
COMMENTS
You’d get an argument around here…
soljerblue Tuesday, October 19th at 1:32PM EDT
“It was Darrell K. Royal who originally said, “When you get to the end zone, act like you’ve been there before” ”
 
Maybe it was and maybe it wasn’t, but around these parts we believe the original quote came from Paul “Bear” Bryant. Maybe y’all have heard of him. Used to coach at Alabama a few years back. Got a few more national titles than ol’ Darrell, or so they say.