If you are close enough to the bride to be invited to her wedding shower, you owe her a personal note, hand-written on an appropriate card without any store-bought sentiments. Keep in mind these cards often are read aloud before the bride-to-be opens your present. Keep in mind, too, that both her mother and the groom's mother may be at the shower, so keep it positive, sweet and sincere.
What Not to Say
Any potentially embarrassing story, no matter how vague you write it, is off limits unless you want your invitation to the wedding revoked. Try not to make a play on the word "shower" as this probably will be done by others. Don't imitate a greeting card with flowery language; keep it simple and direct. According to Miss Manners, never say "congratulations" to the bride. Congratulations are for the groom, as he clearly is the lucky one to have won the heart of your friend or relative.
What to Say
A favorite memory of yourself with the bride when either she or both of your were children should win the guests' hearts. If you are in the groom's family, you may want to mention your positive impressions of the bride when you first met her. If you are meeting her at the shower, write about how you are looking forward to her new place in your life; or you are excited at the prospect of getting to know her. Feel free to write a poem.
If you are attending a shower at the office for a coworker you don't know well, just state you are happy for the bride and you know she will have a lovely life with her husband. Be sure to use both the bride's and groom's names in your note.