Here is our Secret Sister Form that we have the YW fill out. This really helps to get an idea of what the YW like and don't like.
We have purchased a shoe hanger like this one to use. We let the YW decorate their "cubby" with their name, etc. In the extra cubbies, we had paper & pens so they could write thank you notes to one another. We encouraged them to find the opportunity to write to each of the other YW while at camp.
YW can put things in their secret sister's cubby at any time throughout the day, but we scheduled a time each evening to get our mail. The YW sang the Blue's Clue's song when it was mail time! We have had good success with the secret sister program.
If you have a difficult time with equality for your secret sisters (even after setting a price limit, etc.) you may want to do what some wards have done - have each girl make one item for all of the other YW and give them during the week. So, maybe one YW makes a large cookie for each of the other girls and then puts it in their box. Any other secret sister ideas? Share them in the comments!