Just a small addon to thorsail's method - the rescue boat is best if it big enough to stand being whacked by the yacht. In calm conditions, it shouldn't be a problem, but you really want it to stay afloat during the rescue. Obviously it needs a good enough radio to have the range to get there.
If its a small enough pond, and unobstructed, just walking round it trailing a floating line will snag it and let you pull it in. The rescue boat is much more fun, though.
Another trick is to scull the yacht using the rudder. Waggle it about a bit, then leave the rudder set for your preferred new heading. The rudder action can often give enough way to allow a new heading, and since there is almost always moving air even when you thing its perfectly still, the sails might find something useful on the new heading.