During the first few matches with a new teammate, she will compliment you on a good serve or spike. The moment your partner stops giving you kudos, it is time to cheer her up as best as possible. If it is someone you can do without, this is not that important, but otherwise you will need to buy your friend something she will really like to stay in good terms with her. If none of the shops have exactly what you want, go to the Sports Shop and buy a moderately expensive swimsuit. That usually does the trick and usually there is something that she really likes in the shops the next day. Remember, when the two of you break up she will not accept any more gifts from you until the vacation's over.