One would think that a cocktail hour would help, but this is not necessarily so. Again, people form little groups that are very difficult to enter. I know, that when 3 or 4 folks are in animated conversation and I look like I might want to introduce myself the group gets tighter. They don't want to be bothered to introduce themselves or miss out on the gossip. Can't blame them as I'm probably the same way when I'm engrossed in conversation with colleagues.
To help you with your successful event, here are some ideas:
From the Cvent Blog: Set places with questions, not names. Here's a unique idea from Meetings & Conventions' Hot Idea of the Day: instead of labeling place cards with attendees' names, print a question related to the industry, current events or event on them.
Try a centerpiece that is a collage of attendee photos or of last year's events. The collage would be in 2 or 3 mid sized photo frames. (Frames should not be more than 6"-8" tall so everyone can see over them. Very very important. Tall centerpieces are an entire other blog discussion!
Rather than having assigned seating give each person a name card of another guest. They must find that guest and then that guest is their dinner partner. Obviously Jane would have Dan's name and Dan would have Jane's.
THis is a great conversation starter.
There are tons more, but these are inexpensive fun ways to start conversations that aren't intimidating nor would they make anyone feel foolish.