I go to St. Louis maybe 6 times a year from Ames and this is the route I think is the quickest:
Take I-35 to Des Moines
I-35 S to I-80 E (you will hardly be on I-80, don't worry)
I-80 E to US-65 S
US-65 S to IA-163 E
IA-163 E to US-63 S
US-63 S to US-34 E
US-34 E to IA-16 E
IA-16 E to IA-27 S (Avenue of the Saints)
Keep going
You'll join up with the Avenue of the Saints just east of Houghton/north of Pilot Grove.
I'm pretty sure if you Google Maps Ames to St. Louis, this comes up as the quickest way... I think that's how I discovered it myself.
Just realized that this is the same route CYKXBUT posted except, instead of following US-34 to Mt. Pleasant and going south, you cut further down on IA-16 - I think it saves you maybe a few minutes.