How do you get a Yahoo account to even push.
I do not think that you can get it to push.
You need another services like ActiveSync with and Exchange server or MobileMe for the push function to work.
So you need not even turn on push and just set the Fetch frequency to your desired interval or just manualy fetch them.
How do you connect to your Yahoo account?
i.e. POP3 or HTTP?
Either one will not push.
Here are the features with MobileMe. This will explain it all.
http://www.apple.com/mobileme/features/
If you want MobileME you need to purchase this for $99 annually.
Since Yahoo is a free account is your e-mail that important that you need it pushed to you? Most likely not if you are just using a free Yahoo account. So I think that set frequency to a desired interval or just open your e-mail and force a fetch is the right option.