Creating strong poffins in either Diamond or Pearl is ultimately a matter of patience and practice. Collect several berries that you don't care about to practice with, as you'll need to get pretty good at making them if you want to craft enough berries to craft a Milotic or other Pokemon that require a specific trait raise through berries.
Practice specifically regulating the speed at which you stir the poffin, as this will make or break your end result. The best poffins are made quickly with very few or no spills or burns - I think my personal best is a level 2 foul poffin.