Good speaker placement, a good listening position, and then covering the first reflection points in your room with 4" of absorptive paneling from a company like GIK acoustics. Doing so will cut down on room mode interference in your midrange and your high-end (300 Hz - 20 kHz) while dramatically tightening up both the stereo imaging and the time-domain response (ringing/echo) in your room. This applies to your listening position only.

SPEAKER POSITIONING:

In 9/10 cases I would push your speakers against the front wall firing longways down the longest dimension of your room. You can safely ignore most people warning you about SBIR reflections if the topic arises.

With your speakers against the back wall, space them symmetrically between the walls to the left and right of your listening position, but avoid placing them in positions that fall 1/4 or 1/3 of the width between your side walls.

For Example: If your front wall is 8' wide, your speakers should not be placed 2' from each side wall (1/4 of the room width) and they should not be placed 2' 8" off of each side wall (1/3 of the room width). Place them somewhere in between these nodes to minimize destructive interference from room modes.

LISTENING POSITION:

With your speakers against the front wall, place your listening position (your chair) in between both speakers in the spot that is 38% of the room length off the front wall. So if your room is 100 inches deep from front wall to back wall your chair should be located 38" away from the front wall that has your speakers.

Ideally, your listening position should be dead center between the walls to the left and the right of your listening position.

For Example: If your room is 8' wide your listening position should be located in the center, 4' from the walls to your left and to your right.

COVERING YOUR FIRST REFLECTION POINTS:

Covering your first reflection points means hanging at least a 4' x 4' cloud on your ceiling and hanging an equivalent amount of coverage on both the wall to the left and to the right of your listening position. Specifically these should be placed at your first-reflection points.

When ordering absorptive panels don't bother with features like diffusor panels or any hard reflective surfaces. They're mostly pointless. Just get the simple soft absorptive panels (4" thick) and don't worry about the add-ons and up-sells.

Covering your first reflection points will dramatically improve the performance of your listening position from roughly 600 Hz up to 20 kHz. If you mount the panels off of the wall with an air gap equal to the panel thickness it will extend your midrange coverage by an additional octave. So mounting your 4" thick panel on the wall with a 4" air gap behind it improve it's performance by absorbing as low as 300 Hz (instead of 600 Hz). This is good. There is no additional benefit for mounting with a larger air gap.

Locating your early reflection points for your ceiling and side walls is beyond the scope of this reply, but you can look up other explanations of how to find them in your room.

FINAL NOTES:

This (very rough) guide will only treat the midrange and high-end in your listening position (from 300 Hz - 20 kHz) and only for your listening position. Building a room that can manage low-end cancellations and also sound balanced in every position (not just the main listening position) is not a trivial thing, and cost-prohibitive for 99% of people. So you'll have to accept that these solutions are out of reach unless you want to tear down your room and rebuild it.

The fact is, learning acoustic principles to do this yourself is not something that is practical for 99% of people. So don't drive yourself crazy trying to sort it unless you feel very well-versed in physics AND professional audio.

Unfortunately the acoustics space is full of hacks and cargo-cult acousticians who don't truly understand the physics or the practical needs of listeners. It's a lot of voodoo and a great way to waste money unless your designer really knows what they're doing. Unfortunately most people aren't qualified to understand what they need, which makes it that much more difficult to find clarity in an already tricky problem-space. Our team has almost given up trying to make a dent in the field, but the fact is a lot of clients don't know what they don't know and hire people whose expertise is at best outdated and at worst complete nonsense.