The key, is train yourself to look downstream for upcoming likely water and cast at a 45 degree angle to the bank. Because the boat is very rarely going straight down the river, the 45 degree angle should be your relationship to the bank, not how the boat is pointed. For instance, if you are fishing to the left and the boat is going straight downstream use a 45 degree angle off the boat. If the boat is slightly angled to the left, you may be casting towards the left off the bow.
There are a few reasons that this is important. First of all you will get a better presentation. Second you will not have to mend as often and thirdly this will teach you to look downstream occasionally and look for likely areas approaching and give you time to plan you cast. Lastly it will keep both anglers from tangling their lines above the boat in their cast. If both anglers keep the 45 degree rule in mind, you can keep your flies in the water longer and have less tangles resulting in a better chance for more success.