If you want to be sure a girl receives a swimsuit and wear it, you can use the sign-out trick (if you get rid of the latest patch). The latest patch that was released on December 14, 2006 fixes the sign-out trick. If you want to get rid of the patch, go to your memory in the system blade of the Dashboard, and press:
Y, X, X, Left Bumper, Right Bumper, X, X.
You should receive a message that says, "This will perform maintenance on your Xbox 360 storage devices. Do you want to continue" If you click yes, your Xbox 360 will clear it's hard drive cache. If you click no, you'll be returned back to your dashboard menu. This will help your Xbox 360 run a tad faster and fix freezing issues. Note that this will delete all non-Arcade game downloaded content and updates, although your save games will remain.
To do the gift-giving sign-out trick, give a love gift to the girl on the first day, and play against her in Volleyball at least once. Next stay in the same hotel that she is in. That night give her a love gift. Then during each set, put on the suit you want to give her, then select the suit to give to her. During the "Now Loading" screen, press the Gamertag button and get ready to sign-out. If she accepts, she will spin into the suit. If not, sign-out. Then try again with the next suit you have in stock.
It does work. I played Kokoro and gave all of her suits to Ayane in about 4 hours time. This is partly due to the fact that Ayane, Helena, and Tina don't like to be gift given in the Morning set (so I had only Daytime and Evening sets to give suits to). But all the other girls you can gift give during the Morning, Daytime and Evening sets. Then every night, gift the girl a love item from Jack-Of-All-Trades.