Geometric Shapes
Build your towers in sections using geometric shapes such as squares, rectangles and triangles. Triangles are extremely strong shapes, and so are conjoining cylinders. Squares are not as strong, but triangular braces between them will strengthen them.
Strong Base
The key to making a strong tower is building a strong base. A solid base will support a taller tower. Again, use a solid base of triangles or cylinders joined together for the strongest possible base. Triangular prisms are the best bases. Add extra braces to your base pieces.
Equal Lengths
For towers using dried spaghetti between the marshmallows, it is crucial to cut or break the pieces of spaghetti to identical lengths when working on a tower. If the spaghetti pieces are different lengths, they will cause the tower to bend and warp and weaken it.
Tension and Compression
Although marshmallows are great for handling compression, they will stretch and break under tension. Consider running the spaghetti or toothpicks all the way through the marshmallows to avoid having them slip out. Spaghetti doesn't handle tension or compression well, and will break easily. Think about using shorter pieces at the base of the tower to avoid breakage, then longer pieces near the top where there is less compression.