McAllister, who experienced Wembley heartache as Bristol City skipper under Gary Johnson in 2007/08, has helped his side to home and away wins over Uwe Rosler's Brentford this season.
And the 35-year-old believes the Glovers need to treat the final like a trip to Griffin Park if they are to clinch a hat-trick of victories over the Bees.
"What we are trying to do as a squad is to treat it like another game and prepare as if it was any other game," he told Press Association Sport. "I think that is the key, you have to try and do that.
"We've beaten them twice this season but it is about who produces on the day and who handles the situation and the occasion the best. You've got to head in relaxed and look to play your game.
"It is a big game at Wembley but it is just another game for us and we want to go and win it. It is a great day and a great occasion to get there only if you go and win it.
"I've been there and lost and it is a horrible feeling. For me it is just another game."
Having spent four years under Johnson at Bristol City, a spell which included that 1-0 Championship play-off final defeat to Hull, McAllister knows exactly what has made the 57-year-old such a success in his second spell at Huish Park.
"One of the reasons I came here is the way the gaffer gets the group of boys to blend and gel and work together," he said. "He is great at picking out personalities to make the team a unit and as one.
"That is a great ingredient this year, we are a small squad of lads that all want to win things and are all ambitious and hungry for success."