Heads
Heads that are new are essential. Your snare drum is the most used (abused?) drum in your set and is tuned to a relatively high tension, compared to your other drums. You should be changing batter heads every one to three months, and snare heads at least once a year for optimum results. This will vary, depending on how tight the heads are, and how aggressive your playing style is.

Snare Strands
Just 'cos they ain't broke - don't mean they're working right. This is often an overlooked part of a snare drum, but it is responsible for a large part of the sound of it, so it had better be right. The main problem with the snare strainer is that it applies pressure unevenly to the strands, so some will stretch more than others. This uneven wear can cause problems such as snare "buzz" or "dead" snares. When in doubt - change it!

Preparation
Remove the old heads and snare. Clean  the hoop and bearing edges, removing dust, stick debris, and any build-up that is found. Cleaning and polishing should be done now, as there are no tension rods, hoops or snare in the way. Do not use any liquid (your snare drum is wooden, remember?).

Note: If you have any damping mechanism which presses a felt pad against the batter head, remove it completely now! These items are bad news, causing more problems than they solve. If you must dampen your snare drum, use e-rings or other methods. More on damping methods in a later article.

Fit New heads
Place the head onto the drum and spin it around the bearing edge to ensure clean contact. Tighten the tension rods until they make contact with the hoop. Do not tune yet.

Batter Tuning
Put the snare flat on the floor. This deadens the snare (bottom) head, allowing you to isolate the batter (top) head.

Following the order in the diagram below, begin tensioning by turning each key rod 1 complete turn, then continue using a quarter turn at a  time, until the lowest possible pitch the head will resonate a is reached. Tap around the circumference of the head listening for high and low spots and correct accordingly. 

For the modern "tonk" sound everyone loves, the batter head should be tensioned as high as it can go, short of choking.

Snare Head Tuning
Flip the snare drum around, dampening the batter head on the floor, and tune the snare head to approximately the same pitch as the batter head to start.Then tune to the desired tension.

The way you tune your snare head controls the sound the snares produce - looser, and you will get more snare "wash" - tighter, and the snare sound will be more controlled (think military snare drum).

Experiment a bit to find the piches which work for you, try a looser batter with a tight snare and vice versa. Or try both heads loose or tight. See what sounds best.

Snare Strand Tuning
Once again, experimentation is the key here, but as a general guide: The looser a snare strainer is, the more it will work, and the "fuller"  it will make the sound of the drum, as well as adding sustain. Tighter, will give more controlled sounds, but be prone to "buzzing" sypathetically with other drums or instruments. If you can hear the pich of the drum change as you tighten the strainer, you are tuning it too high.