It's tough to keep everything warm and serve at the same time, especially for big events. The first thing I recommend is getting various size stainless catering pans. They have lids, are stackable and easy to set in the oven to keep warm. I got tired of borrowing crock pots and roaster pans and all the cords that goes with them. Once you get groups over 30, crock pots get pretty small and useless.
The main thing I strive for is to make sure all the sides are finished early as it is easier to keep baked beans or corn warm than it is to manage 15 racks of ribs.