Where are you having the problems? Most rulers do have a 45 degree angle mark. Some days I have an awful time getting the angle the direction I want, other times I do just fine.
I do a lot of pinning and when I am coming to where I want to stop I put in two pins, stop your seam allowance away from the edge, or sew to the edge and pick out the last couple of stitches. I start by cutting the first angle making sure it is exact before pinning.
There is a lot to be said for measuring your quilt and cutting the borders to fit. But of course, I don't do it that way! I cut my border pieces long and pin about every two inches and I don't have to worry if my seams were running a bit scant or a bit generous.
I have pictures that show that I use 1/2" seams for the actual miter, allows me a little give and take and cutting off that extra fabric is scary! I draw my seam lines with pencil on the wrong side of the fabric.
Now, if the issue is matching the designs in border prints it becomes a lot more complicated. Basically you have to match each corner to the design, and I always get at least one that doesn't end up the way I want. So I've designed quilts to fit within the border fabric by a small plain inner border. I've done it so that two of the corners match and the other two don't... I've done all sorts of things depending on the project.
Edit: It also helps to make sure your corners are square first! Sometimes it is the details that get us
