The marketing opportunity to be had at the moment you find you have been unsuccessful in winning a bid
If you have put in a bid, particularly for something complex such as extensive professional services, you will have had a lot of contact with the potential customer.
In the first instance, it may have taken you a long time to find them and the process of bidding may also have been time consuming as you find out more about their requirements and the professional solution that you might have to solve their problems. It is also possible that you will have incurred considerable costs.
How do you put emotions aside, consider things from your customers point of view, and see what marketing advantage you can gain from it?