For the invitations we put "Follow the Yellow Brick Road to ..." and then the details. Actually we didn't give too many details. We called it "A Special Night of Sisterhood."  (If we had let the sisters know it was a Visiting Teaching  conference, the attendance would have been very poor. This way we piqued their curiosity.)

Our evening went REALLY well. We did a dinner with all the centerpieces and table ware in black and white. Then we went to the RS room for 3 ten minute classes. I wore a blue gingham dress and carried a stuffed animal and introduced everyone. The sister missionaries on Courage (how to make that first contact and work with inactives), a mom of 7 on Love (she has always done her Visiting Teaching in all stages of life despite kids, home schooling, teaching early morning seminary, etc), another sister on Brains (Visiting Teaching with intelligence, she is a busy professional with a non member husband who always finds time to do her Visiting Teaching and always gives the lesson). Then we followed the yellow brick road, stickers on the hallway, back to the cultural hall for dessert. The cultural hall was now decorated (thanks to the bishopric) with balloon bouquets for centerpieces and multicolored table settings for rainbow sherbet and cookies with m&ms.

The sisters got graduations diplomas (brains) in different colored papers and tied with different colored ribbons. There were RS stickers in the lower corner of the diplomas. We attached rainbow charms (from www.charmingldsgifts.com ) to the ribbons. I told them to attach the charms as zipper pulls to their wallets, scripture cases, purses, whatever, where they would see them everyday and remember to do their Visiting Teaching. They also got chocolate hearts (love) and medals (courage) made from chocolate coins and ribbon.
 

Some additional comments from Donna:
 

Oh, by the way, we did one other thing that night that made a real impression.  Since our dinner only used half of our full size Cultural Hall, we set up tables only and we filled the other half with a snake-like line of chairs.  My husband helped put them up; it was quite a job.  We put up one chair for each sister on our rolls: almost 200. 
 

I taped a card to the back of each chair with a sister's name in alphabetical order.  The sisters found and took their chairs when they got there.  Even though we had a good turnout, around 40, it didn't look like we used more than a few of the chairs.  The visual effect of all the missing sisters was very dramatic.  Well worth the time and effort.