This tip may seem obvious but I have only just started doing it. If you have a brew you like to make over and over this tip is essential in order to produce consistent quality and taste every time.
I recently re-brewed my Flowers Original Clone (updated the recipe to a Partial Mash). In the original recipe we used 0.5oz (.25oz for a half batch) of Target hops. The alpha acid for Target is typically in the 9-12% range. In the original recipe the hops I used had an alpha acid value of ~10%.
So when I came to re-brew 6 weeks ago I noticed my new batch of Target hops were 11%. So I decided to go back to BeerCalculus at Hopville (and my spreadsheet) and recalculate the IBU’s. At 11% alpha acid for the bittering hops I calculated I needed to reduce the amount of the Target hops from 0.5 to 0.4oz. Good job I did because the ale is in the keg and it tastes absolutely fantastic