Lawnsalon gave good advice.
Flowers beds are too labor intensive to be profitable in many cases. Timing, proper soil conditioning, watering, adding nutrients, replacing dead fowers...ugh.
I have always had better results with tree and shrubbery beds instead of flowers. Flowering trees (weeping cherries, etc.) always get applause this time of year.
Know your zone and know what works best in your soil. Be selective in your approach of shrub selection to get coloration through the year. Hollies with heavy berry growth give a nice touch, nandina get colorful in late summer, barberry show red through the summer. Add bulbs for color intensity.
Pick plants that fit with your project. Don't force a plant where it doesn't belong. Think about long term growth...don't plant a tall growing tree under a power line. Learn about landscape design and follow some simple concepts; symmetry, triangular plant spacing, three level display.
One of the hardest parts of landscaping is educating your customer. Go in with a plan and a solid understanding of every plant you plan to install.
Take before and after pictures.
Lawn Care Business