The first rule of investing is preservation of capital. The second rule of investing is having an appropriate allocation strategy that encompasses an array of investment asset classes, otherwise known as diversification. Are there more rules? Yes, but none more important than these two. Preservation of capital, diversification of investments; both rules apply to real estate investing as well.
Preservation of capital is imperative because without it there are no future investment decisions to make. Having an allocation strategy is helpful in assuring your basket of investments maintains a reasonable level of balance between investment types; not becoming over-weighted in a single category.