If you're a homebrewer who uses liquid yeast from Wyeast or White Labs, one of the steps you can take to ensure healthy fermentations is to make properly-sized yeast starters. In general, the size of your starter will be proportional to the gravity of the beer you're brewing. The Mr. Malty Pitching Rate Calculator will do the math for you.
A drawback of yeast starters is that they dilute your original gravity and hop bitterness. Thankfully, it's easy to compensate for a yeast starter in your recipe*. Let's say that you want to brew a 5 gallon batch with an original gravity of 1.060 and hop bitterness of 60 IBU. If you typically lose a half gallon of wort in your fermenter, your total fermentation volume should be 5.5 gallons. According to the pitching rate calculator (with the drop-down menu set at "Simple with O2 at Start"), you should make a 2-liter yeast starter. Because one gallon equals 3.785 liters, your yeast starter will be (2 liters)/(3.785 liters/gallon) = 0.5 gallons. To calculate the volume of your main batch that should end up in your fermenter, subtract the yeast starter volume from the total fermentation volume. The resulting value is 5.5 gallons - 0.5 gallons = 5 gallons. If you lose half a gallon between your kettle (measured hot) and fermenter (measured cold), your post-boil volume (measured hot) should be 5.5 gallons.
You can figure out the target gravity and bitterness of your main batch by rearranging the mixing formula:
Aa + Bb = Cc -> a = (Cc - Bb)/A
Capital letters represent volumes and lowercase letters represent either specific gravities or IBUs. A/a represents the main batch, B/b represents the yeast starter and C/c represents the combined "wort" in the fermenter (the formula treats the yeast starter as an unfermented liquid, which is necessary to determine the effective original gravity of the beer). Assuming your yeast starter will have no hop bitterness and a specific gravity of 1.040, the target gravity and hop bitterness of your main batch can be calculated like this:
OG = (5.5 gallons x 1.060 - 0.5 gallons x 1.040)/5 gallons = 1.062
Hop Bitterness = (5.5 gallons x 60 - 0.5 gallons x 0)/5 gallons = 66 IBU
When you plug the new numbers into your recipe software, your grain and hop bills should automatically be adjusted. To maintain a consistent color and flavor, I recommend tweaking your percentages of specialty malts and flavor/aroma hops so their weights remain the same. That way, your yeast starter simply replaces some of your base malt and the IBU correction is achieved in your bittering hop addition.
*Geeks: the most accurate way to do this is by calculating the total masses and extract masses of the two known quantities, and then using the relationship between specific gravity and degrees Plato to calculate the volume and gravity of the third quantity. However, for low-gravity liquids such as wort and yeast starters, the the simplified method described above will result in smaller margins of error than your measurement methods can detect. Where mass analysis becomes essential is for post-lauter additions of high-gravity liquids (e.g. honey) and solid adjuncts (e.g. corn sugar).