I-20 to I-30 around Dallas then 287 to Amarillo to I-40
Sea Level Ram is correct about this. I-20 loops south of Dallas and is much better than taking I-30 through town.
For my part, I take a wider berth around DFW. I get up to US 82 at either Paris, TX or Sherman, TX. You have to go up that far north to avoid the congestion in the routes around the Metraplex.
Just to give an example of this strategy: you could take US 69 near Tyler all the way to US 82 at Bells near Sherman. US 82 will merge with US 287 at Henrietta, TX and on to Wichita Falls.
But, if you want to go via Dallas, stay on I-20.