First, if you don't need the liquidity find investments that you will get paid a premium for taking on illiquidity. Second, look for alpha or return enhancements where you can find them. Hedge funds can be alpha enhancers and substitutes for fixed income or beta. Third, cut your public beta exposure or at least change it to more illiquid investments that can give more beta per dollar invested. Most private equity will be at higher market beta or credit beta.
This endowment liquidity sensitive approach is not for everyone. You have to be able to sit on investments for long periods without a need for cash and you have to have skill to find the alpha enhancers or private investment managers. Yet, it is worth thinking about how these portfolios can be replicated in a different more accessible form.