I struggled to keep a consistent 1/4" seam and it was really a combo of cutting, thread and stitching. So now I use my Accuquilt as much as possible and otherwise do as Tartan showed, I use a thinner thread (a #50 or higher) as that really makes a difference, and take the time to check the 1/4" seam when I start a project by cutting 2 accurate 2" blocks and sewing together and then checking that my new rectangle is 3.5" exactly--if not I adjust my needle position--and I find that on my machine I always have to bump it a bit. Also, on those 1/4" foot with a flange, which I use, don't be a "pusher"--what the machine repair shop told me! Was told it's a guide, not a curb to bounce off of!
Good luck--I tell my beginner quilters that the hardest thing is to get that 1/4" seam consistently--once that comes along almost everything else--including Y seams, seems much more doable!