It is still VERY risky to plant at this time of year. Much of Indiana has to wait until mid-May for the average frost-free date, and even then, that is only an average. Frost can still come much later than that! My area of Northern Indiana has been known to get frosts in June, and on very rare occasion, July! So, the fact that I planted 3 flats of flowers yesterday, may not have been particularly smart. But, I have lots of sheets for covering them in the event is does get cold again.
One thing that I like to do when planting anything is to make sure the roots aren't too root bound before planting. You'll know if a plant is root bound by taking the plant out of the container and examining the roots. If they wrap around the root ball of the plant, then they are root bound. The petunias I bought yesterday were a little root bound (see roots wrapped around the bottom of the root ball), so I needed to break up the roots a little bit before planting them. Breaking up the roots is called "teasing." I prefer to do this by hand, but other people like to use a tool such as a pocket knife.
The general process of teasing the roots went like this: First, I gently pulled on the roots that were wrapped around the plant. I ended up breaking some off and tearing a few in half. This is perfectly fine. In fact, breaking some of the roots can help to stimulate root growth after it is planted. Of course, breaking too many can kill the plant! After gently tugging on the roots, I spread them out a little bit. I set the plant into it's hole, and gently filled the dirt in around it. I used my hand to pat the dirt lightly to prevent the plant from falling over. Stamping on the ground with your foot around your plants can compact the soil in such a way that it hinders a plants ability to drink and breathe.
I have planted many flowers with teased roots and un-teased roots, but the ones with the teased roots almost always perform better in my flower beds. The ones that weren't teased were especially likely to pop out of the ground if they were disturbed by critters or kids!
Here is a great reference from Purdue University for when and how to plant many popular annual flowers. http://www.hort.purdue.edu/ext/HO-080.pdf
And here's another Purdue publication with more detailed planting and care tips. http://www.hort.purdue.edu/ext/HO-99W.pdf