You're not supposed to put it in a bag before you put it in a mason jar.
The reason mason jars are used is because they are pretty much airtight and the glass doesn't pull the trichomes off due to an electrostatic attraction like plastics often do. If the buds have been dried correctly, no condensation will form and mold won't be an issue and the bud will cure whilst it's being stored, but it's still a good idea to 'burp' your jars periodically by opening them up, dumping your bud out and carefully putting it back.
Some people have suggested that for long term storage removing the oxygen can extend the shelf life of your bud. Whilst you could put an inert atmosphere in like they do with perishable food products (nitrogen being the most commonly used gas), an oxygen absorber used for long term food storage would be more feasible. All these are are just cheap sachets of powdered iron with salt and sometimes some activated charcoal. You can make these yourself but they're so cheap you may as well buy them.
There is another simple way of storing your bud - vacuum sealing. Lots of people I know say this works really well, is cheap once you have the equipment and is easy. The only drawback is it crushes your weed so it loses it's bag appeal but if you're the only one smoking your bud this won't be an issue. One piece of advice I would give would be to invest in the proper equipment as cheaper ones don't work well enough.