Since mattresses are not replaced very often, it can be hard to keep them smelling fresh and clean. But there is a quick trick you can do to keep them smelling fresh.
Next time you change your sheets, remove the mattress cover (it is good to wash those too!) simply apply a light dusting of baking soda. Then put your mattress pad and sheets back on the bed. Next time you change the sheets, simply vacuum up the baking soda from the mattress.
This works great on kids beds when they have had wetting accidents or if pets decided to do their business on your bed (cats seem to love to do this when they get mad!) However, if this is the case, you may want to consider a washable waterproof mattress cover, which will keep the mattress from getting damaged from accidents.