Prepare to brew something with a bit higher gravity, then measure before you start the boil and adapt/rebalance from there.
After making a lot of medium-high gravity extract brews (~6-8%) I wanted to make some medium-low gravity (4-5%abv) beer in my first two all grain batches (I now have a kegerator, and I tell you, 8% beer on tap in your living room is not a good idea). Botched the sparging horribly in both cases, and ended up with ~50% efficiency (was shooting for 75%). If I were brewing something heavier I could have just cut a bit on the hops on the recipe, adapted, and still come out with something decent. But since I was aiming low, in both cases I ended up with what would amount to <1.030 post boil, and basically had to top off with extract to avoid ending up with Bud Lite.