Planting hanging baskets

I've always had a problem doing these properly so they grow into a lovely rounded ball of colour. Today, I half-filled a 12" basket with compost then half-way up, I inserted some non-trailing lobelia then some trailing plants which I wanted to grow around and amongst the lobelia which should help support each other. After pressing their root-balls down and inserting the stems of the trailer plants through the upper basket wires, I added more compost to cover their roots. I then added more compost to fill the basket and put a trailing fuschia in the middle. That has left me with an outside area around the Fuschia to fill, but I'm going to be planting stuff on top of the previous plant rootballs. I have a 16" basket yet to plant up, so need some advice.