I believe that, realistically, there's little you can do.
Holding some cash so that you can buy at when prices are down may improve your Sharpe ratio but will reduce your expected returns, which you presumably care more about.
I believe if you look closely at the cost of using options you will find it is far higher than the expense ratios you would otherwise encounter, and it makes that route unattractive.
A cheaper (but more dangerous) option would be to get leverage using index futures, however I would not do that either. Once you go over 100% equities you usually open yourself up to risks that don't exist if you stay at 100%. It's not worth it.
I think 100% really is a sweet spot, for a certain type of investor, it doesn't make sense to go below it, but it also doesn't make sense to go above it.
Your best option might be to have a flexible mortgage and a investment account, and rebalance between them. Any credit available on your mortgage account counts as "cash" to the extent it give you buying power for equities, but may not count as "cash" from a psychological point of view when you consider whether you are reasonably maximising your returns, as even a 100% equity investor can consider a paid-for house as an attractive objective. (Note that any debt used for leverage must be non-callable, if this type of mortgage is an option for you, read the small print.)