A good drawing program like Coreldraw or Adobe Illustrator, or even better Aspire . Enter in node editing mode and make sure all lines a joined properly at the corners.

below is a sample when you magnify a problem. Third pictures looks ok but could be problematic if you have not joined the lines:

Click image for larger version.  Name: lines.PNG  Views: 253  Size: 4.7 KB  ID: 19605 Click image for larger version.  Name: lines 2.PNG  Views: 279  Size: 4.6 KB  ID: 19606 Click image for larger version.  Name: lines 3.PNG  Views: 235  Size: 5.7 KB  ID: 19607