Events are a good way to strengthen relationships with customers and prospects alike in a semi-informal setting.  What makes a good event?  It’s the trinity of good food (a buffet facilitates networking better than a sit-down meal), good location (your premises would be preferable, but if you’re located somewhere out in the sticks, or only have a small site, consider hiring somewhere a little more convenient) and some kind of quirky feature that encourages your attendees to talk about your event with their colleagues the following day (and to attend your next one!).  Keep it professional, though – the office Christmas party may make for an alcohol-fuelled merry old time, but customer events are not that party.