Let's assume that you have your permit to climb Half Dome, or are planning to climb on Monday-Thursday when no permit is required, and want to be ready for the 17-mile hike up and back. What do you need to do to prepare?

Train — Training should start from the fitness level where you are. In general, you should begin walking/hiking at your normal rate and increase your speed or distance or difficulty of terrain or weight carried in a pack about 5% a week. It's safer to change one variable at a time and make changes gradually so that you don't risk injury. (former article)

The climb from Yosemite Valley to the top of Half Dome starts at about 4,000 feet and ends at 8,842. Forty-eight hundred feet is a lot of elevation gain and loss for one day. Train in the hills.

If you are starting from scratch (you don't hike on a regular basis), it would be advisable to check with your medical provider to see that you are a good candidate for a strenuous hike.

Gear — Wear clothing made of synthetics because of their wicking ability. Plan to wear layers that you can peel off or add back on as temperatures shift during the day.

A hat with covering for the back of your neck. Sunglasses and sunscreen. A bandanna has multiple uses including added sun protection. Although hiking poles might be helpful on much of the trail, they could be a nuisance on the 400-foot section with cables. If you take them, have the collapsible ones and a way to carry them.

Properly broken in (not worn out!) boots. Bring blister bandages or Moleskin or other covering for hot spots or blisters on feet (and a small knife for cutting the Moleskin if necessary).

Gloves for the cable section (REI or similar outdoor gear suppliers carry lightweight liner gloves). Sometimes you can find discarded ones at the cables; in a pinch you could use socks to protect your hands.

Trip planning — If at all possible, go to Yosemite a couple of days before the climb. Even though most people do not experience altitude sickness at this elevation, the lesser amount of oxygen will affect your energy level. Going early gives some time for acclimatizing to the altitude.

Timing — Start your hike early when the numbers are smaller.

Water — Carry plenty (1/2 to 1 gallon). If you carry water-purifying pills, you can top up your bottles at streams along the way.

Snacks — high-energy, low weight items such as peanut butter or cheese and crackers, nuts and seeds, energy bars, GORP (Good Old Raisins and Peanuts) sports gels, etc.

More info on how to prepare for the climb 

Reminder: If you are planning to make the hike on a Friday, Saturday, Sunday or holiday this year (and probably 2011), this is the schedule for obtaining the required permit:

First day to make reservations, (7 AM, PT)
for May - March 1
for June - March 1
for July - April 1
for August - April 1
for September - May 1
On-line: www.recreation.gov (recommended)
Hours: 7 AM - 7 PM Pacific Time (November through February)
7 AM - 9 PM Pacific Time (March through October)

Phone: 877/444-6777
(or 877/833-6777 for TDD)
(or 518/885-3639 from outside the US & Canada)

My previous article, Yosemite's famous Half Dome hike to require permit, part 1, has lots of additional information on the new regulation.