I learned this idea from a friend a few years ago and it has been a huge blessing. She had given us some glowsticks and I asked where she got such an assortment. She said "It was something we had in the birthday box. I don't remember where we bought it."
The idea is to keep a box of gifts that you can give out for birthdays. It may be my own children's birthdays or their friends birthdays. Now when my children are invited to a birthday, they just pick something from the birthday box and wrap it up in paper or a bag that is right next to the birthday box. The bags are all used, so hopefully we don't give a present in the same bag that child gave to us but if so, eh.
Yesterday we went to Target and they had a "Buy 2 get 1 free" on games. So we bought some and they went into the birthday box. B is interested in one of the toys so we may keep it for his birthday. lP has a friend that may or may not have a party tomorrow. I still haven't heard anything about it except rumor, and I know that family was sick on Wednesday. But we've got a present ready just in case.
I was never on the ball before. We were always trying to rush to the store and pick out something on the day of the party. Now it's wonderful to just choose a present and spend the time making a card or having the child wrap the present. It shifts the responsibility to them a bit and I like that. And it gives me the ability to just buy nice gifts I see on sale, without an anxious child at my elbow insisting that their friend wants whatever cheap plastic thing they actually want for themselves.