Aim, hold and squeeze only gets you so far
Everyone learns to shoot at different levels, and we all basically figure it out at some point, but the fundamentals for shooting will always be the fundamentals. There always little add ons that we can do when trying to improve our shooting skills and for those familiar with competitive shooting, without using a sandbag or a bipod, a good rifle sling can give you far more stability than just a good grip. You have to learn the positions you need to get into and how to adjust your sling to the correct length, but I have watched people improve their off the bench accuracy dramatically. You will get very good windage control if you are setup properly.
A new era of rifles are upon us, the M4 Carbine is probable the most commonly accessorized rifle out there, but there are some really well thought gun slings for it. Blue Force gear rifle slings are probable our biggest sellers and have lots of good references from those in the sandbox.I always suggest quick detach slings because being that the M4 Carbine is meant for CQB, you may get in a situation where you are getting snagged or someone gets a hold if you and detaching yourself from your gear may mean getting away. You may have to do some slight modifications to your AR15 when you switch your traditional rifle sling over to a different configuration.