Make it fun and mix it up, but don't abandon those passes you can make at 34 (or what ever speed you want to make you maximum speed).
Run a set through the rope lengths and take a crack or two and 28 at max speed, and then a couple 1/2 to 1 mph slower. 28 off is certainly a line length that you can go out at slower speeds (30 and 32) to get a feel for the rhythm so try a set of 28's too.
You should also "own" 22 off, so I think there could be some merit to running several in a row (3 or4) and working on the fundamentals that will help you at 28 off; like vision, rope control and body position.
Free skiing at 28 off is also a good idea. I like to free skiing a slightly faster speed (i.e. 35mph instead of 34 mph), but that's a matter of personal preference.