Did you use trailing petunias? Those look a bit more upright. I'd go for the ones that are specifically labelled trailing. I've had the same disappointment after mistakenly using regular petunias. They just don't look as nice imo.
I use water retaining granules and plant feed granules from Poundland 
Water and deadhead daily.
Look up thrillers/ spillers/ fillers re. hanging baskets.
I had great success with the hanging basket 6pk selection from Homebase last year. They're labelled by colour (pink, purple, white off the top of my head). I had the pink and purple ones. They contain a vigorous trailing petunia (rather hardy, mine have survived the winter
), a fuchsia, bacopa, creeping Jenny and a daisy for example. If you plant one pack in the same container they look fantastic, really complementary. I think Aldi might have them in at the moment. Haven't seen them in Homebase yet but suspect they'll be on sale soon (May).