1. Total body coordination - swimming is not just about how well you kick or how good your arm-stroke or how you breath with ease. It is a total body workout that needs harmony. You may have a correct arm-stroke but still be a lousy swimmer. I think the roll over drill popularized by Alexander Popov can help you feel the water and with the help of a videocam, you can correct your stroke.
2. Make your butt float - do not sink your butt it will cause a lot of drag. Focus on this and it will be easier to tackle long distance swimming.
3. Front sighting - navigation is important in open water swimming, practice front sighting just in case you have to do this on race day to look at the turn around buoy.
4. Long strokes - do not be lazy on going for the full length of your stroke, start with a long catch and finish it. When muscles fatigue sometimes you sacrifice long full stroke with short and fast, avoid this during training.
5. Roll over drill - did I just repeat myself? Remember: harmony.