1. Increase the drag on a single scull or other boat by wrapping bungee cords around the hull. I find that a single cord astern of the cockpit works pretty well in the single. For more resistance you can use multiple cords, put the cords in front of the cockpit (you will get wet!) or even tie a long rope with an empty coffee can on it to the rigger and let it trail behind the boat (better for use at low ratings).
2. Row against the wind – no brainer.
3. If you can get a headwind that opposes the current on a river or tidal estuary, row against the wind but with the current – this will increase the load on your blades at the catch because you are essentially levering the blade against the flow of the current. This only works in a noticeable way if the headwind is strong enough to slow you down significantly between strokes so that the boat speed is low relative to the current at the catch.
I find that it is impossible to muscle the boat to go faster, because the boat slows down enough between strokes to make it impossible for a human to overcome the inertia. You have to find the balance point at which you can maintain a smooth power curve with maximum acceleration, and the boat will go 5-10 seconds faster per 500m than just pulling all out. This translates well when you take the bungee off, but it’s easier to feel the most efficient power curve when you have some resistance on the boat.
Today I was able to have conditions 1, 2, and 3 above, and I did the 1500m in 6:47 at a rating of 28, which is about a minute slower than I would normally row it.