This is a little off topic, but I also like to use a double needle for straight line stitching. I prefer the wider width of double needle. Works great sewing through the diagonal of 9 or 4 patches, whether big or small. I have both a queen sized and a small 9P patches sewn that way. First I hand basted the large quilt. In this case I started in the corners and stitched a big X in the quilt, then worked out from the middle to the edges. No walking foot,but use the open toe foot; no wrinkles in back or front.

are you using a walking/quilting foot? and a longer stitch length, like 2.4? that should help.

We call it a walking foot, the industry calls it an 'even feed' foot. Talk with your local dealer if you have one and let them know that it is the foot that 'walks along the fabric'. The Pfaffs have one built in and is called IDT. For years it was only the Pfaff that had it built in, but now there is another one or 2 that offer it on their machines too.

Sorry, I have a hard enough time knowing the English version of words so Dutch is way out of the my line of knowledge :).

The post previously about the "even feed foot" is the other name for the walking foot. On the Janome it is called the dual feed foot. All of these names are the same in principle. When you sew a seam, the fabric right against the feed dogs is pushed along toward the back of the machine and the top piece is moved with it. Sometimes you will notice that the top piece ends up being longer than the bottom piece. That is because of the action of the feed dogs. With an even feed foot, it's like having a set of feed dogs on the top fabric and the set on the bottom fabric. This way the top and bottom get evenly fed through the machine.
When you start to quilt, you need to stablize the quilt sandwich by doing the line of stitching from top to bottom and side to side. And then you can work in the four quadrants.
Or, start in the very center and quilt your way out.

always stitch from the center out...

As was mentioned in a similar topic, one reason for puckers in the back, is if the foot pressure is too tight ...

http://www.youtube.com/watch?v=Ox8qRhWF3CI

Read the former posting Help, please. I believe your problem is related to this one.

Try using a PVC quilting frame - example and free instructions at: pvcworkshop.com

many other examples on google

I do a lot of straight line quilting. I never stitch from the center out. I do start with the center lines but I start at one corner if I'm doing the diagonals and I start from the same side for each line. I used my Pfaff for years and now I have a Janome with a dual feed system.

Other than a walking foot or even feed system of some type the real secret is getting a good baste. I have tried spray basting and pin basting and for me the very best method is the Sharon Schamber thread basting method. She has a couple of videos on youtube. Just search her name and they'll come right up.

in the first machine quilting class I took, the instructor told us to use safety pins - spaced about the width of your hand or 4 " apart. We used painter tape to tape the backing down on a flat surface, spread the batting on top and smoothed it many times, then placed the top on that and started in the center and pinned until it was done. My layers didn't shift, and I didn't get any tucks.