I'm working on a cocktail menu for a local bar and restaurant on the Peninsula (more on that later) and I've been preaching to them the idea of pre-mixed drinks. High West and Germain-Robin are selling pre-mixed cocktails in the bottle. Many bartenders I know in the city use pre-mixed cocktails in their own repertoire. It's a way to prep in advance, yet offer customers something special at the same time. Letting the ingredients of a Manhattan congeal slowly over time helps to meld the flavors together. David OG went to a bar in London once that sells one-year old Manhattans – cocktails that have been marrying together in a glass bottle for 365 days! Crazy, right? There's no reason you can't do this at home. Do this – make a batch of your favorite aromatic cocktails (not citrus-based) and leave them to sit for at least 24 hours. Then make a fresh version of that drink and try it with the aged version side-by-side. See if you can notice the difference. I've got rye Manhattans, brandy Manhattans, Calvados Manhattans, Negronis, and plenty of other concoctions sitting on my mantle right now. One thing - do not add the bitters in advance. You need to do that when you actually mix.
See what you think. Pretty soon you may be pouring a bottle of gin, Campari, and sweet vermouth into a giant jug the moment you get home with them. Why wait when those components could be melding together right now!