Sorry - I didn't explain very well. I meant plant the repeat plants together, in groups rather than scattering them through - your eye wants to see and register the similarity.
There are two ways of doing this: you can simply put them next to each other to create a more vibrant area of texture/colour (this is probably the easiest way). The picture here is a result of doing this - this is Beth Chatto, characteristically planting in clumps so she gets that amazing variety of colour and texture really punching through.
Or you can drift them through a border, focusing on repeats. I think this is more risky, though, for two reasons. Firstly, you need to be fairly constantly on top of things to make sure they are all happy all the time. Secondly, it can look like a dog's dinner if you aren't really careful - you seem to need an awful lot of repeats for this to work (think twenty salvias rather than three, each of them perfectly placed). The second picture is a brilliant example of achieving this high art - it's Patrick Collins's superb garden at Chelsea.
Of course, you can mix the two up a bit!!
I don't know why but I find odd numbered groups work best for me visually - three seems to be ideal, five for a larger drift. Anyone else agree about this?
You can get the same effect by buying single plants, but it takes years and years to divide them and get them to the right size!
There is probably a whole technical language to describe this - apologies to the true professional landscapers on this forum for my amateur attempts!