I installed something called Gutter Filter in my new house eavestroughs.
Works pretty good for two summers now. Google will find it.
It is a triangular black plastic spongey material that you just push into
the trough. It is a bit over-sized so its own springy pressure holds it in
place. The triangular shape leaves some of the flow area open for water
flow under the filter.
Leaves just sit on top and blow off. The only problems are at roof valleys
where dirt and small seeds build up on the top of the filter and partially
block the flow - I haven't seen any overflowing gutters but I went up and
blew the crap off with a leaf blower; only took a few minutes.
We have a lot of very high oak and elm trees and get a ton of leaves so
these gutters would have been murder to keep clear without something.
The filter is very coarse so fine sand would get through but nothing
larger. It's not perfect but way better than nothing.
Our winters are very cold and I chose this system because I figure it might
be better at dealing with snow and ice. Not sure if others wouldn't have
been as good.