Ideas you might consider;
Color: Grouping seems to have greater impact than mixed planting.
Fragrance: A GREAT added benefit from beautiful roses.
Size: Planning pays, don't have a big rose up front obscure your
smaller ones.
Spacing: Good spacing keeps many ills at bay. Air flow stops mildew.
Type: Many or few, Three Mr. Lincolns in a grouping is to die for!
Loose, well watered compost rich soil and you are home free ;-)
JTillery