The "backwards Z" method is an easy way to find common denominators of two or more fractions. Have a pencil ready.

[edit] Steps

List 2 or more fractions with different denominators. We will use 1/2, 3/4, 4/8 as an example here.
Draw a line to the right of each fraction. Each line will be the central bar for a new fraction.
Find the least common multiple of all the denominators. In this case, it's 8.
Put the LCM under the lines you drew in step 2. The LCM is your new denominator.
Starting with the first old fraction, draw a line from the denominator to the 8 in the new fraction to its right. Realize that 2 x 4 = 8.
Draw a diagonal line up to the numerator of the old fraction, and multiply by the same number you did for the denominator.
Put your result in the numerator of the new fraction. In other words, 1/2 is equal to 4/8.
Do this for each fraction.

[edit] Tips

There is always more than one way to do anything in math, so if this doesn't work out, you can try another way.
Make sure your answer makes sense. If you somehow converted 1/2 to something ridiculous like 3109/1324, you need to go back and check your work.