My colleague Julie Gelo, a foster and adoptive parent in the double digits, shares these tips for helping children in foster care transition to an new family:

Have the current foster parent request photos of the new family to show the child frequently [or make a "transition book" using the custom picture book tip].
Ask what kind of laundry soap and fabric softener the new family uses and have the current family start to use them (or have the new family use what the current family is using for at least a while).
Have the new family send a blanket, stuffed animal, scarf, etc to the current family so that the child can start to use it now and take it to the new home with her.
I would have the new family attempt to use as many of the same comfort items as possible that are sent with the child and not be in a big hurry to change them over or to wash them.
Have the families exchange information on schedules, foods, soothing techniques, favorite music, favorite books, toys, and so forth. Are there certain lullabyes, games, and so forth that the child finds soothing or comforting? What do they use for eating utensils, does the child sit at the dinner table in high chair, or booster seat? As much information as possible that can be shared the better the transition.
In an ideal world I would have the transition last 3-4 weeks with short visits in the current home, then out in public with both parents there, then at the new home with the current parent present and so forth.
If that's not an option, make sure to have the exchange of information, use of photos now and in the new home to maintain connections, and the use of common items, smells, textures, foods, schedules and so forth.

To which I might add some more rudimentary ideas that somehow seem to get overlooked:

Unless the child is in immediate danger, moving a child without preparing them ahead of time is NOT a good thing. By ahead of time, I mean well before the car is pulling up in the driveway. Losing a familiar family and home without preparation is truly traumatic.
Never ever never move a child by putting their things in garbage bags. Kids are literal, and "Your belongings are garbage to me" is hardly a nice message to send to a distressed child.