Garnishes are a tricky issue. How many should you keep around? If your guests claim they don't want a paper umbrella or tiny plastic sword in their drinks, are they telling the truth or just pretending?
Love for paper umbrellas is nearly universal, but there are also perishable garnishes that your guests crave, such as green olives or pearl onions. These should ideally be kept in the refrigerator at all times -- you never know when or how someone will want a martini. The same goes for fresh limes and lemons, which are used in hundreds of drinks. Slice them in wheel form to use as a garnish, ream them for their juice or peel them to use their zest. Other garnishes, like pineapple wedges for tropical drinks and celery sticks for Bloody Marys, aren't essential for a home bar, though you'll probably want to stock them for special occasions or themed gatherings.
One last garnish to consider is fresh herbs. Mint and basil, for instance, are easy to store and will add a powerful flavor punch to many cocktails.