present a problem for the young retriever. The tendency to "square" an entry throws a dog off line and complicates marking. Once a dog knows how  to handle, this natural tendency can be reduced by de-cheating which teaches the concept of taking the correct line.   

To further work on angle entries and exits, I use short tune-up drills. Short is good because there is more control over your dog when you are close. Repeated exposure to the concept 1) makes the dog more aware of what you are trying to teach 2) establishes permanent expectations and 3) produces a comfort level which makes it easier to enforce. Here is a pond with two drills set up to deal with left and right sided heeling.  This drill is not a major part of a day's training and not done every day. 

While training on these grounds,  I usually have a land and water series of marks. The tune-up drill (if we need to work on this) will be run in between. The water set-up would incorporate marks with angle entries and exits (while it is still very fresh). I'd rather handle in a drill than on marks, but if handling is necessary the concept is clear and easy to enforce.   

When first setting these up, each "line" will have two visible white bumpers, and we work for precise lines. Later it becomes kind of a mini-set of pattern blinds and the bumpers are changed to orange. Finally, it becomes a review of the concept (kind of like a maintenance drill) which can be done in various ponds of similar design.  As you can see the distances are short (can and should be increased for the experienced dog) so it doesn't take long to run.  No e-collar is used because it is all teaching through attrition.