For long only trend following systems, I know of some people who use index filters.
Some have them set for if the index is in a downtrend, their systems turn off, for some this means they don't buy anymore but keep all current positions open until an exit is hit as per normal. For others it means they sell
Though I must say these index filters are not universal in their approach. For some long term trend following systems, this approach gives no advantages, as its hard to know when to get back in, and often by the time you are back in, you'd have missed the aggressive rebound, so the net result is breakeven.
I speak only of long term trend following systems because that's where I've done my research.
Nizar.