A simple tip -- if you're going to have food laid out on a buffet table, especially before a talk, don't put the table flush against the wall unless you absolutely have to.
Rather, move it out from the wall, so that there can be two lines (one on either side of the table) instead of one. This will speed things up dramatically, and let the non-food part of the event start more quickly. A very simple suggestion, but I've noticed that many organizations neglect to do things this way.
A second tip, if you implement the first tip, though this requires a little more foresight: Make sure that you have two serving utensils for each platter or bowl that requires serving utensils. The two-line solution will still work even without the double utensils, but it will work better with the double utensils.