At formal 'wedding-style' buffets, guests get called up table-by-table; you should go to get your food promptly when requested, as guests who are slow off the mark can hold up the conveyor belt.
At parties with a buffet, timing is less crucial, but it is a help to the hosts (and caterers) to keep an eye on the rest of the room and eat with the majority rather than holding up the clearing of the main courses and the serving of pudding.
Move along the buffet at a reasonable speed to maintain the flow, and be polite and accommodating to waiting staff who may be on hand to serve. Be aware of people behind you and offer to help where necessary, either by handing them a plate, knife and fork or napkin, or by serving them where practical. Take plenty, but avoid overloading your plate or finishing dishes that look like they may not get replenished. Usually there will be the opportunity for second helpings, but multiple visits to the buffet table may look greedy.