the suggestions to let some air out of the tires are good.

You can do certain things to dampen the road buzz transmission on your bike - you might want to think about how much money you want to invest in parts on a bike that you're already planning on upgrading at some point.

There are buzz kill vibration dampeners that you can stick in the ends of the road bike handlebars - those aren't too expensive, I don't know if they fit into flat bars.

You can add carbon to the bike you have - a carbon seatpost will dampen some of the vibrations to the seat. YOu can put on a carbon stem or a set of carbon handlebars - these are things you could swap onto your next bike if the geometry is similar or you stick wtih flat bars.

You could also try switching to specialized zertz parts - specialized puts these inserts called zertz in their bike parts to try to dampen vibrations - you can get handlebars with that, and I think seatposts. Maybe stems. Ebay's a good place to buy pieces that people have swapped off their bikes and that should be relatively inexpensive.