This event tip is fairly straightforward; before you sign off an event get a confirmation that the person who sold it to you will be present on the day.
Event companies tend to be set up in one of two ways. The first way is that you have one point of contact all the way through the process; this contact will prepare your proposal, be your point of contact in the run up and be there on the day. The second way tends to be adopted by larger companies who departmentalise; they have a sales team, an organising team and a delivery team. Both ways work, and there are very good reasons why the companies who adopt different procedures do it that way.
However, there is a potential problem area. As companies become larger and more target driven it's possible that some members of their sales team may be prone to making what might be described as glib promises to make a sale. This is particularly the case when the primary target focus is financial rather than quality. Because of this there is often conflict between sales and delivery teams. This isn't unique to the event industry, it's a fairly common business issue.
In some industries, particularly those in which delivery requires qualifications or specific skills, it's not appropriate for sales people to deliver. In events the interpersonal skills required for sales and delivery are fairly similar, so there is no harm done if a sales person works on an event. They may not be the right person to manage it, but they can contribute. Therefore it's quite acceptable to ask that they are present on the day.
If you make it clear from the start that this is your expectation then it's must more likely that the salesperson will take more care with the initial construction of the event. This isn't a general criticism of all salespeople, but rather an observation based on 25 years of event experience. If a controversial point were to be made it would be that, in most cases, the larger the company and more financially target focused it is the more of a problem glib promises are.