The best tip I can give is to invest in a quality silicon mask which fits well around your face. The plastic masks typically sold in the beach stores usually fill with water very fast and make it impossible to see. A snorkeling mask with a wide glass area is also best for snorkeling since it offers a wide angle of view. The snorkeling masks that have two lenses -one for each eye- are better suited for deep diving but restrict the field of view. A good mask should cost over €35 and you can find them in outdoor gear stores, or specialized scuba diving stores. A good shave is also prescribed for male divers since a beard cannot let the mask create a seal around the face, leading to a mask full of water and a frustrating snorkeling session.
If you have ever been snorkeled you probably know that lens fogging can be a frequent problem. Surfacing and cleaning the mask every five minutes can be vexing and taxing. There are a variety of products in the market that promise to keep a snorkeling mask clean, but I have found saliva to be just as effective. Before entering the water rub some of your own saliva on the inside of the lens and rinse it with sea water. If you do this before the mask gets wet you should have a fog-free lens for at least half hour or more.
For a long time I used an inexpensive €2 snorkeling tube which worked fine, but once I invested the €25 to buy a tube with a filter, I enjoyed my dives even more. The snorkel with the filter allows breathing even when small waves crash over the tube, and I would recommend them highly.
A pair of quality fins can also enhance your snorkeling experience by allowing effortless propulsion through the water, and by keeping their shape after many many dives. Expect to pay over €40 for a good pair of fins, and you can purchase them at the multitude of scuba diving shops in Greece.