Go light! No more than 10% of your body weight should be on your back so say those who have been there and done the walk. What follows are details of what I carry/wear. Some of the items my packing list can be found here.
Boots are the most important to get right. The boots I have are Scarpa SL3 leather walking boots. They are very sturdy and being leather took a bit longer to wear in. Its worth it. I initially had a few problems that were solved by doing a few shorter multi-day walks to break in the boots. It is important you do this and not begin the camino with new boots! Poor quality boots may not last the distance.
Socks should be good quality walking socks that allow perspiration to wick from the foot. Coolmax fibre I find is good. I have 3 pairs – wear one, wash one, and the emergency pair. The local outdoor store, Tiso of Scotland, has a good selection available in store and via on-line shopping.
A pair of walking trousers from the likes of North Face or Craghoppers that zip of to make shorts or 3/4 length trousers (or both) is an excellent idea and saves on weight. They also dry quickly and are light. Three sets of underwear should be enough if you can get into the habit of that regular evening wash. Swim wear is also useful in hot weather.
Two tee-shirts and a shirt. A Poncho (a Trekmates Typhoon or an Altel Atmospheric poncho are highly recommended) for wet weather, especially those that cover your ruck sack too. I finally decided on the Ferrino Trekker RP L/XL. A fleece for chill evenings and mornings is a must.
A wide brim hat will keep off the rays on the hotter days.
Accommodation along the route is not a problem except maybe in peak season. As I started in August, I decided to take a lightweight tarp (MSR E-Wing single person tarp) and bivi bag (Rab Survivor Zone Bivi). It’s a good emergency choice should that auberge be full. A sleeping mat (Therm-a-Rest Z-Lite) is always useful and is also very light.
Bring a good rucksack that you have used several times on long walks. I have an Ortovox Peak II 42 litre that has straps to reduce it’s volume if carrying less volume – very flexible and great back support. It makes a big difference if your rucksack is comfortable and does not swing around. I also have two walking poles which I find helps to take some weight off the knees, improves balance, and can be used as tent poles with the MSR E-Wing tarp if sleeping outdoors.
Some of the above can be found here.