The first thing you need to understand is that a sphere full of liquid will NEVER fly perfectly straight. There are things you can do to increase the accuracy of a paintball however. First off, you need to have a good ball to barrel match. The flatline is a larger bore barrel, around a .692. So, the first thing you need to do is get some paint that fits well in your barrel. I reccomend RPS Super Swirls or any line of Nelson Paintballs. They seem to fit the best. Also understand that if a ball as any impurities like dimples or brittle shells, they are least likely to travel in a straight path. Then there is the issue of porting. A good paintball barrel is solid for the first 6-8" followed by about 3-4" of porting. The Flatline does not do this. Along with the porting problem the Flatline is more difficult to clean and can sometimes have a better chance of ball breakidge in the barrel due to the backspin that is induced from the curved barrel. 

I personally am not a fan of the Flatline Barrel System and would suggest that you look for something a little more like what was explained above. However, to each his own.

Eagle Keep 

Eagle is sort of right in that the problem is most likly related to the paint you're using.

The flatline is sensitive to both paint and velocity.  Using larger bore paint will not allow airflow around the ball to help impart a spin.  And if the velocity is too high the ball will tend to spin less.  Now, knowing this, the two best ways to increase the accuracy of your flatline is to use small bore paint and keep your velocity around 260-280fps.  Also keep in mind that depending on your set up and playing style, one brand of paint may not give the best results even with the same type of marker.  This is why when someone asks what paint is best for my marker, you get people who will say it's the best and others who say it's the worst.  Dialing in your marker for maximum accuracy is a trial and error process, don't be afraid to try somthing new or different.

Poisoner wrote: I don't expect the balls to be flying perfectly straight, but I did not know the Flatline had a bigger bore.  I will try some bigger paintballs and getting the low pressure kit too.

Poisoner, if you get large-bore paintballs for your flatline you will be very disappointed.  The flatline is the one barrel for which the paint-to-barrel match rule does not apply.  The flatline has a large bore, but is designed to shot small bore paint.  Everyone in the know agrees that Marbelizer and Evils (though expensive) are the very best paintballs for the flatline.  If you shoot those and it still doesn't perform as expected, the problem may be elsewhere.  But considering how very picky the flatline is on paint, switching to a better sized paint is a good first start.