Some tricks you can do.
Check as you go. If you're working using pencil and paper, every now and then, pick up the paper, turn it around facing away from you and hold it up to a light source. Alternatively hold it up to a mirror although I prefer just using a lamp because it's more easy to move around and seems to give more accurate idea of what you're doing. If you're working on the computer, do the same thing by occasionally flipping the image horizontally. Either of these will point out areas wher eyou need to straighten your lines.
You can also try laying down a very light grid, although I wouldn't generally recommend it- it might help to make the whole page straighter, but also might lead to stiffness in forms as well as possibly still leaving slanted things within the squares of the grid.
Use your editing software to fix things if it's too late to go back. I use GIMP, I'm not sure if it's the same for photoshop. If you don't have editing software, download GIMP, it's free and hasn't given me any problems and can really help make your work look better digitally. What I do is I use the "shear" tool, which allows me to highlight part of a page that's slanted and shift the bottom while leaving the top anchored (or vice versa), pulling it along until the slant has been fixed.
When you draw, do you get your head down really close to the table? Do you look at your work from the left side, since your hand is working on the right? This is part of what causes your problem. When you look at the pag e from wherever your head is as you're drawing, the proportions all seem fine because you drew it from that perspective. But when you move, you notice everything looks warped. Changing the way you sit while drawing may help this.
It's good that you're aware of the slant, because that means you'll be able to work on eliminating it. Many of us struggle with the same thing. Hopefully some of my tips will be of some use to you.
Don't kid yourself, friend. I still know how.
"I'd much rather dream about my co-written Meth Beatdown script tonight." -JSConner800000000