A parked motorcycle is still placing demands on its tyres. If storage lasts more than a month, keeping the same patch of tread under load can produce a flat spot. A centre stand can take weight off the rear tyre; lifting the front requires suitable, stable support. Without that equipment, move the motorcycle a short distance from time to time so a different part of each tyre meets the floor.
Choose a dry, well-ventilated place. Keep tyres clear of fuel, oil and solvents, and park away from heat sources and electric motors. A mild soap solution followed by a plain-water rinse is enough for the sidewalls; harsh cleaners and shine products can damage the rubber and reduce grip.
Before the next ride, use a gauge to check pressure while the tyres are cold and inspect the tread and sidewalls. Cracks, bulges, cuts or uneven wear call for a professional assessment. In cold weather, ride moderately for the first few miles: cold rubber is harder, so traction may feel different until the tyres warm.