The Line:
For most fishing a weight forward floating line will do. Weight forward lines allow for more distance, ease of casting for beginners and turn over flies with less effort. Double tapers give more control, roll cast easier, and present the fly with a more delicate presentation. For getting deep there are a number of sinking lines and sink tips available in different sink rates.

Gear Storage:
The fly fishing vest is one of the most popular ways to tote your gear. There all also a number of chest packs and shoulder bags out to give a more ergonomic method to storage. For boat fishing don't forget a gear bag to keep stuff organized and out of the way.

Polarized Glasses:
One of the most important items to have. Polarized lenses cut glare off of the water allowing you to see the fish.

Waders and Boots:
Chilly water and temps require waders to keep you dry. The waterproof breathable versions are best and can be worn with comfort even when air temps start to rise. In cooler conditions layer with wicking layers or fleece. Felt soled boots are essential for safe and stable wading.

Weather Forecast/Water Levels:
A good one to be aware of especially if fishing from a boat or in the back country. A surprise thunder boomer can get you in trouble if you are not prepared for it. Water levels can mean the difference between a wadable or floatable river and a brown torrent in which fishing can be bad at best.

Professional Advice/ A Professional Guide:
Check with the local shop before you go. Those are the guys that fish the most and know what's going on. They can definitely help with fly selection. If you have the time and money, a guide can make all the difference in the world.

Food and Water:
Make sure you have plenty of water and a snack. It will make your day more enjoyable and will keep you fishing longer.