Dry yeasts can be a great and less expensive alternative to liquid strains. They don't require a starter and they remain viable much longer than the liquid yeasts.

There are, however a few things that you must consider when using dry yeast packets.

1-the viability of that dusty old packet you found in the back of your fridge.
2-you still need to follow proper pitching rates
3-most dry yeasts are designed to be re-hydrated before use.

For 1 and 2, you can look here:http://www.mrmalty.com/calc/calc.html

this handy calculator factors in the manufacture date of the yeast, to tell you exactly how much you'll need to use for your particular beer.

For the third issue, lets go straight to the manufacturer for some advice:

Rehydration And Usage Tips For Ale Yeast

Pitching Rate: 1g/L
Step 1.
Sprinkle the yeast on the surface of ten (10) times its weight of clean, sterilized (boiled) tap water at 30 - 35°C (86° - 95°F)
DO NOT STIR !!!
Leave undisturbed for 15 minutes at 30 - 35°C (86° - 95°F)
Foam or no foam is not an indication of vitality

Step 2.
After 15 minutes stir until all yeast is suspended
Leave undisturbed for another 5 minutes.
Adjust temperature of solution to that of the wort in 10°C (18°F) steps, by adding small amounts
of wort at 5 minutes intervals and mixing gently (ATTEMPERATION)

Step 3.
After attemperation inoculate without delay.
Aeration of wort is not necessary.

That's directly from danstar. I'll add that i like to do this in a pint glass of microwave boiled water, that I cover with foil immediately after heating. You can keep the now steam cleaned foil over the glass with the yeast in it so that dust/dirt/nasty bugs don't fall into your yeast while it's rehydrating.

So once you've figured out how much yeast you need, wait about 20 minutes before you plan to pitch, then rehydrate the yeast packet(s).