What do you do after you beat psaro the manslayer in dragonquest4?

Answer 1

For the NES version: You're done. Nothing else to do.

If you're playing the PS or DS version: Well, after the credits you save your game. Then, you will go to Rosehill and find out that elves are living under the Yggdrasil tree. Going to the Yggdrasil tree should give you clues that mention that Yggdrasil Flowers have the power to resurrect those beyond the power of Yggdrasil Leaves (like Rose!). However, those only bloom once in a millennium. To get one, you need to clear the bonus dungeon located in Azimuth (defeating the pair of bosses at the end; careful, they're pretty tough).

Once you've done that, go back to the Yggdrasil Tree and get to the highest floor. The flower should be somewhere in the southern part of that highest floor. Grab it and zoom to Rosehill. Use the flower on Rose's grave to bring her back to life. She begs you to take her to Psaro, so Zoom to Heaven's Haven and go back to the site of the final battle. Psaro will make to attack, but Rose will intervene and her tears will reverse Psaro's evolution.

Now, back to normal, Psaro joins your party. With Psaro in your party, you can go to Diabolic Hall and fight Aamon. (You need to have Psaro in your party or the fight won't trigger)

I have a nice article on how to beat Aamon at: http://hubpages.com/hub/How-to-beat-Aamon-in-DQ-IV

After you beat Aamon, you've truly completed Dragon Quest IV. Very few things change in the ending, but I won't mention what does, so it's not a spoiler (although this whole answer is one huge spoiler. ;)