Plan a party.
When we added a very large addition to our house (over 1,000sf addition), we did much of the finishing work ourselves. Well, when we began to get burnt and slacked off a bit, we would plan a party. This was great motivation to get some things done on a deadline. Worked every time!!
It worked so well in fact, that we did the same thing, plan a party, to get some deep cleaning underway.
But now I find it is much easier to do a little cleaning every day so the house always looks presentable and I don't have to look over my shoulder to see the condition of my house, should the door bell ring with unexpected company.