There isn’t a whole lot of information on “proper etiquette” in the way of bridal shower invitations. It is definitely not comparable to the plethora of information and rules we have regarding wedding invitation wording.
Of course, you want to keep with the theme of the wedding. For instance, if the wedding is to be very formal, you would want your shower wording to be formal. A ladies luncheon and tea shower is obviously a bit more formal than a mixed gender shower with a backyard barbeque.
You can choose cutesy wording that rhymes or just very straightforward who, what, where, when wording. Keep the information relevant and light. There is no need to overwhelm your guests with a lot of extra information.
If the bride and groom are registered, this would be an appropriate time to let them know of that. Most businesses make a little enclosure card that you can just casually slip in with the invitation. If your bride does not have any or you are surprising her with the shower, you could indicate either on the invitation or a separate cards, i.e. “The bride and groom are registered at Bed Bath and Beyond” etc.
At times, it is a good idea to indicate if food will be served. Especially if the shower is at an odd time like two o’clock in the afternoon. If the shower is at noon, guests will most likely assume they will be served at least a light lunch, just like your guests would assume a dinner for a five o’clock shower.
Don’t forget to include a phone number of whomever is throwing the shower, not only for guests to reply, but also incase guests need additional information like directions, additional gift suggestions or a dress code idea.