Another useful principle is to make the good bits look really really good, rather than try to do anything with the less good bits at this stage.
So deadhead, weed, cut off all yellowing leaves or part eaten leaves, broken stems etc in the key areas. There may be time to buy in a few plants to add interest and colour, and a good covering of mulch (see yesterday's post) will set it all off.
Don't panic buy! It's tempting to go to the garden centre and just take what's in flower! I've bought some foxgloves and epimedium I wanted for the garden anyway, and couldn't resist some penstemon. Often such plants in full flower look impressive in the nursery, but are just swallowed up and look insignificant when placed in the garden (this is the case with the penstemon). A large pot full of such plants, placed well, can be a better option at this late stage. On the other hand if you see just the right plant - go for it - buy as many as you can afford!
Cast a critical eye over the less good bits of the garden and do the absolute minimum there. Throw down some dark mulch. The idea is to have nothing there which catches your eye, so take out the yellowed leaves, broken branches, leaning stems....It should be a neutral space which the visitor's eye just glides over in it's search for the interesting bits.
Finally remember that no-one else is likely to look at your garden with the same critical eye you have been seeing it with these last weeks - most people will simply enjoy the fruits of your labours and think how lucky you are to have such a beautiful garden!