Exchange MC for C-Bills, or buy a 'Mech bundle. 'Mech bundles include three 'Mech variants of the same chassis, three 'Mech bays, premium time and some include cockpit items.
Buy a 'Mech Bay, which allows you to store more 'Mechs.
Buy premium time, which increases your XP and C-Bills by 50% for every match. Purchase from 1 day to 1 year.
Buy consumables using MC or C-Bills. Same price as if you bought them via the MechLab.