A very experienced quilter once told me that sewing a really straight seam was one of the most difficult things to do in quilting! That said, I use one of those feet with the little "rudder" on the side for 1/4 in. seams and it helps. Going slow and steady also helps, as does angling the fabric slightly to the right as it starts feeding (someone mentioned how the feed dogs tend to pull to the left). These things help but I still have to watch very carefully as I'm sewing and always have the trusty ripper handy.
I'll keep the "post it" suggestion in mind now too since sometimes the fabric shimmies under the little rudder on the 1/4 in. foot.
I really liked the comment from the person who said that her seams are straight but her rulers are "off"!