Before you spend a day in a new pair of leather sandals, strap them on, and quickly step into and out of the pool, or a bucket full of water, and wear them damp. This will help the sandals to soften up and conform to the shape of your feet, it's better to first wetting a tiny test patch in a hidden location to make sure there is no discoloration, then toweling off the excess water after the sandals take the plunge (they should be slightly damp, not waterlogged). If the sandals are delicate or expensive, forgo the baptism and stick with a traditional leather moisturizer.