Honestly, sewing a straight 1/4 inch seam had me crazy until last night...until I found and watched Judy Martin's video. After trying a square on my own and having to evaulate and correct I took time to find tape and place it correctly.I placed black tape at the 1/4 ' spot ( it does not appear correctly on the machine or when using the 1/4 foot) so the tape does help. I've used many ideas before, but this one seems to work. Give it a bit of study to see if it can work for you.
Judy Martin
http://www.judymartin.com/video-page.cfm
(sorry I could not make the web)