I use tube traps in the corners. Depends on the size of your room and speakers as to how much improvement you will get.
Acoustic panels and big bass traps are expensive. Clearly one way is to get a company involved. After the corners, I experimented with heavy blankets and different pillows… couch pillows are good to determine what was needed where. One of my earlier rooms actually sounded best with painted walls. This was with ribbon speakers. One foot high three foot wide panel of heavy wool oriental carpet six feet up the front wall made a great improvement.
I have a very large irregular room that sounds great by itself, so I did my corrections with heavy wool hangings and pillows… with a few official acoustic products. Lots of ways of approaching it.