Most fin manufacturers will have their own buckle / strap design. The simplest and most common one allows you to pull the rubber strap on each side to tighten the fin. A buckle or quick release allows you to loosen them quickly and take them off. Prior to the next dive, you would quickly extend the rubber and lock the buckles again, ready to be used again.
There are a number of spring straps available, allowing you to exchange the rubber straps. This makes the donning and removing of fins much quicker and simpler. The spring keeps the foot in place, but you will need to replace the spring straps after a while, as they do tend to loosen eventually.
Helpful Hint: If you go on a diving holiday, we recommend you take a least a pair of spare straps with you for your fins.