Peruse the safety category for suggested items you ought to carry with you, including water and a first aid kit. Don't forget to pack a camera!
Boots and Footwear
Well broken-in boots are a necessity, particularly for long hikes. In early spring and summer you may have to travel on snow and deal with high-water stream crossings, so water-proofed boots are important. You will also encounter rocky sections, so you will want good, firm footwear. You may also consider wearing thick or double socks for more support.
Some people like to use hiking sticks or poles to help on trails with particularly steep descents and steam crossings.
Carry an extra layer of clothing in your day-pack to avoid hypothermia; it can happen even in the summer. For some trails you may wish to wear pants and long sleeves in order to avoid brushing up against stinging nettle.
There are numerous creeks, ponds, and lakes with water all year. The water from all of these sources is likely to be contaminated with giardia at best and other things at worst, and if you need to drink from them you must treat the water either chemically with iodine water or peroxide tables, or with a water purifying pump.
Animals and Bugs
It is particularly delightful to see a moose feeding in a lake or pond, but it is always wise to give them plenty of room. On rare occasions you may run into a rattlesnake; give them room and it is unlikely they will give you any problem. Use insect repellant with DEET to deal with insects, and check yourself occasionally for ticks.
First Aid Kit
It is always wise to carry a first aid kit that contains some basic items like antiseptic, various size bandages, aspirin or other mild pain pills, an Ace bandage, antihistamine, mole skin and blister treatment items, as well as a pamphlet summarizing treatments for fractures, snake bites, serious cuts, etc.
Food and Water
Remember to eat and drink often as you travel; trail bars and the like can give you a boost; fresh fruit tastes better and will do the same thing. Generally there is no potable water on the hikes, but there are a few springs to stop and fill your water bottles. Some people find that electrolyte replacement drinks help them on long, hot trips.
Map and Compass
The hikes described on this website are are generally on well-defined trails, and in the cases where the trail becomes indistinct, the use of a USGS map and a compass or GPS will keep you from getting lost, particularly if you are not familiar with the route you will be using.
A good sunscreen and a brimmed hat are essential, and you may want sunglasses. In early spring, if you are traveling on snow for any distance, sunglasses are cricial to protect you from snow blindness.
Fire-building is discouraged. Backpackers should use portable stoves rather than use scarce wood. In the event that you become lost in the canyon, be careful if you need to build a fire; there have been instances of lost people starting large forest fires.
Cutting Through Switchbacks
Do not make a cut-off on swtichbacks. The switchbacks were not built for tired hikers but rather to prevent erosion of the trail. A cut-off can erode a gully several inches deep in one heavy rainstorm.
The size of your hiking group should be small enough to allow all members to enjoy the experience without undue noise and dust. Too many people on the trail may scare even a moose.
"Leave No Trace"
Leave the trail better than you found it by carrying out your own gargabe and any trash you encounter along the way.
Preserving the Environment
Remember you are a caretaker of the mountains, not a despoiler. Use common sense to preserve future enjoyment. Though carving your name in a tree may bring excitement to your own life, it creates ugly marks that are a distraction to others who wish to enjoy the natural setting.
Yielding on the Trail
Since many types of recreationsists use the trails, it is important to recognize the yield triangle: mountain bikes yield to hikers and horses, hikers yield to horses, and motorized vehicles yield to all.