Situated in the Mediterranean, Greece is a pleasant country to visit throughout the year but there is a huge difference in temperature across the seasons. Therefore, it is advisable to pack according to the time of year and type of trip you will be taking.
In summer, the heat is often scorching, especially in the south and in the Cyclades. It is thus essential to protect yourself properly from the sun and to take a hat, sunglasses and high-SPF sun cream. Try to choose light clothing and avoid synthetic materials that encourage perspiration (and bad odours).
If you envisage visiting religious monuments or the Metéora, don't forget to pack a long skirt, trousers or sarong if you are a woman in order to cover your legs and shoulders and, if you are a man, make sure you pack trousers. For the beach, take plastic shoes that will protect you from bear cubs and stones, as well as a waterproof camera to take photos of the fish and coral. Greece has many snorkelling spots in the Cyclades. It would be a shame not to immortalise those moments.
If you are going in the low season or to the mountains, a pullover or a jacket will be very valuable. The temperatures there are much cooler than in summer and in the south. Likewise, a raincoat is essential, especially in autumn and spring when it rains regularly... in downpours! Even then, remember to take sunglasses. The light is very strong in Greece and you would quickly get sore eyes (and a sore head) without protection.
If you are planning on a trip through the Greek islands and getting around by boat, ask your chemist for a box of seasickness pills. The wind can blow very strongly, particularly in the Cyclades, and sensitive people could get sick. Keeping in mind that some trips last 6 or 7 hours, it is better to prevent seasickness than battle through it!
And if you forget something, don't worry: you should be able to find everything you want in Greece!