There are several ways to improve a FoS:
• Changing to a stronger material.
• Increasing the material thickness.
• Increasing the radii on the internal corners.
• Increasing the overall dimensions of the part (perhaps 40x40 instead of 30x30).
• Radically changing the design of the part (think of solid bar instead of box).
• Change the design of the assembly.
Sharp internal corners will give a theoretical infinite stress. The smaller the element size in the analysis, the greater the stress at these places will appear to be. Include fillets on the model to overcome this problem.
If the FoS is 1, the item is on the point of failing (with the specified loads). If the FoS is less that 1, the item will fail. A FoS of 3 means that it is 3 times stronger than it theoretically needs to be. This is a normal minimum for FoS to allow for unforeseen circumstances and any difference between the theoretical and the practical (as these may not match exactly). A very high FoS is OK but you might want to review the design to reduce weight or cost.