Remember that the overall rationale of diversification is to survive as many "what if" scenarios as possible. The bonds in a 80/20 portfolio are there to prevent you selling too many shares during a recession. The international allocation is there to mitigate the risk of a US-specific problem (similar to the Japan problem mentioned above). Think through the sorts of scenarios you might have to survive and pack accordingly: inflation, deep recessions, Japan scenarios, rising or falling interest rates, commodity or housing bubbles, sovereign defaults, liquidity crises, etc.
Also consider the many diversification options that are not binary stocks/bonds, like preferreds, REITs, MLPs, options funds, commodities funds, convertables, etc. These all zig zag differently than the stock market, but can be just as painful to watch at times.