I used to have the same problem until I read Sally Collins "Mastering Precision Piecing". My problem was two fold ... one was in starting the seam, the other was in continuing the seam.
When I started a seam I would put my foot down, put the fabric up to the edge of the foot and start to sew. Sally taught me to always start with the needle in the down position, lower the presser foot, raise the foot ever so slightly (enough to get fabric under but not enough to disengage the tension) and butt the fabric all the way up to the needle then slide it over to the right until it's at the correct 1/4" position, THEN start sewing.
When sewing a seam, especially strips - guide the fabric with your left hand (splayed hand - thumb in front of needle/foot - little finger in back of needle/foot), and feed the fabric with your right hand with the fabric slightly raised off the bed of the machine in front of the foot.
Also ... I too use a 1/4" guide but I always adjust my needle position to get the right seam allowance.
Finally ... I use a 1/4" walking foot almost exclusively when I piece. I have a Janome 6600 with the built in walking foot, which was one of the biggest reasons I purchased that machine. I absolutely love it. The walking foot helped, but I still had some issues with uniformity of my seam allowance until I employed Sally's techniques.