Always have a back-up plan. As mentioned in Tip 5, weather may not always permit the intended party plan. In these cases, you should have a great back up plan or be ready to see a very unhappy child on their birthday and a houseful of kids with idol hands.
Have an alternate location picked out in advance if your home will not accommodate the entire guest at one time should the party be rained on or other situations arise. Regardless always make sure food is mobile, and you have plenty of back up games up your sleeve!
Do your best to readjust the party to new environment in a relaxed manner. Getting uptight will be picked up on fast and the young guest will react accordingly, which is never pleasant! As long as you appear to be having a great time, the guest will too.