Supposedly the golden shovel makes fertilizer more effective. I've only had the shovel for two days, but I got some new hybrids from it the next day - one black lily and one pink carnation the first day and one purple tulip the next day.
I bury the fertilizer and have 4 pairs of different hybrids around it, like this (F = flower, X = fertlizer):
FFF
FXF
FFF
That way, I have a chance to grow 4 different hybrids with one fertilizer. Make sure there's space for flowers to grow around where you plant them. I pick up some of the flowers before I dig the hole, so that I don't accidentally dig up a flower.