If your computer is older / slower, sometimes the brush tool might not always be the best choice. It's possible to use the pencil tool if you plan on reducing the final artwork in resolution by at least a factor of 25%. Otherwise, the brush tool is your best bet for smooth lines.
Slower machines might cause the brush to lag or hang a little, potentially making your lines lose curvature. I have a problem with RF interference that causes the cursor to jump a little, intermittently. If this is happening to you, make sure your computer speakers and / or CRT monitor(s) aren't located too close to your tablet.
Last of all, try not to draw with your wrist but rather move your entire arm when drawing longer lines. This will help you draw smoother lines both on a tablet and on paper.
Hope this is helpful
