First, choose a big lake that gets partly frozen in winter, which is liquid in summer. This lake reflects much of the sun's heat that lands on it back into space, when it's frozen, but absorbs sunlight when it's liquid.
Drop into this lake a tall heat pipe, whose internal fluid has a freezing point
lower than water's. The bottom of the pipe should rest in the lowest point of the lake, and the top should stick just a bit above the surface. Preferably, the pipe should be at a diagonal, so that the top isn't directly above the bottom.
When the air temperature drops below 0C, we would normally expect such a lake to have ice at the surface, 0C water just below the ice, 4C water at the bottom of the lake, and a thermal gradient between the coldest and warmest water. Since water is at it's densest at 4C, no convection occurs.
With the heat pipe, however, heat from the 4C water is carried up to the surface, where it warms the 0C water. As the 4C water cools, it rises. As the 0C water is heated, it descends. Since the heat pipe is at an angle, this creates thermal convection, which will continue as long as some of the water in the lake is between 0 Celsius and 4 Celsius.
Once all the water in the lake gets to 0 Celsius, the convection stops, and the water at the surface freezes. As this occurs, water along the length of the heat pipe will also freeze. Ice will spread through the lake, both down from the surface, and outward from the heat pipe.
The lake with the heat pipe will take longer to start freezing than if it didn't have it, but once freezing begins, it will happen faster and more thoroughly than if the pipe hadn't been there.
As a result, the lake will stay frozen longer, and won't thaw until quite some time after it would have thawed, had the pipe not been there.
If heat pipes can be cheaply placed in many such lakes, lots of the sun's heat can be reflected into space.
Furthermore, if we choose thermokarst lakes, which release methane to the atmosphere when they're liquid, we can slow global warming in two ways -- by reflecting more sunlight, and reducing released methane.