The best way to increase accuracy is to increase consistency by using high quality fresh paintballs. Accuracy is mostly reliant on the consistency of the paintballs you use. Low end cheap paintballs may be very inconsistent in size and may have dimples or oil that can greatly affect the accuracy.
It also helps to use a barrel with a bore that matches the paintballs you are using. Because paintballs vary in size with brand to brand, batch to batch, and the weather it's best to have a couple different bore sizes or a bore kit.
If the barrel you use is too big for the paint then too much air escapes around the paintball in the barrel. If the barrel is too small then the paintball gets pinched causing friction and barrel breaks. Ideally you want the bore to be just slightly larger than the paint you are using.