This is generally harder as the wedding party gets larger, but in general:
If you want them all in the same dress, try to pick something that will be flattering across different body types (assuming your friends are not all the same approximate shape).
Consider the same dress in several different colors - for spring weddings, pick 5 pastel colors and each attendant gets a unique color that they can select with the groomsmen matching the bridesmaids. For black tie, jewel tones can work. This lets each woman pick the most flattering color option. I was in a wedding where we were in dresses that got progressively darker blue that made for really pretty photos (bride in white, attendants in sky blue, periwinkle [me!], and navy).
Consider selecting a line of dresses that coordinate or picking a skirt and providing a selection of tops. Many Designer Bridesmaid Dresses & Gowns lines will have options in the same color and fabric but different cuts that allow each to pick something flattering. You can ask for something like "full length in burgundy taffeta by XX designer" and pretty much anything in that line will work nicely together.
Decide that the bridesmaids being in a "bridesmaid" type dress isn't important to you and ask them to select (and run by you before purchase) a dress in a narrow-ish range. Something like "blue knee length dress" gives the bridesmaids the ultimate freedom to find something that is flattering and in budget and possibly better odds of wearing it again.
Remember that these dresses can be crazy expensive and most will need alterations so consider your bridesmaid's financial situation before selecting a $350 dress that can only be worn once.