In my game, I've found that the athletic skill merely aids the progress. My acrobat is almost at level six of her career and only has two points in the athletic skill. Having your sim pratice the tricks either on his own or while playing for tips will probably help more.
This probably ties in to the fact that there seems to be a hidden skill level for every trick that rises each time you perform it. For example, let's take that fire stick one. The first time your sim does it, he'll wave it around in the air and attempt to spin it. He'll drop it and maybe catch on fire. But the more times you perform this trick, the better your sim will get. After a while, your sim will be able to toss the stick in the air and spin it across his arms.
Also, on a side note, the skill level for each trick seems to be separate. I love the fire stick thing but hate juggling, so my sim still fails a lot when she juggles during a show, even though she has a higher career level than required to perform the trick.
I should probably note that these notes are merely based on my observations, and that I'm not sure if anything in the game data supports this.
