You can do this by investing in index funds such as various mid and small cap indices.
There are more asset classes than just mid- and small-cap indices. One can go high level - such as commodities - to enumerate asset classes, or go granular - grains/industrial metals/precious metals/etc. etc.
It will then be important to be continuously rebalance.
That's easier said than done. While technically anyone can create a spreadsheet to show them how to rebalance, or maybe even when to rebalance, I've yet to see people that do this on their own on a systematic basis. There are programs out there that will offer quarterly, semi-annual, or annual rebalancing, while balanced mutual funds will generally not go by timing, but rather allow assets to fluctuate within bands, and rebalance when out of band. It's important to find a manager that actually follows what they say they will do.