Our style of schooling (almost unschooling) doesn't tend to lead to lot of summer slide, at least when it comes to the "unschooling". That part of our learning is life directed, and doesn't really stop for summer - if anything it picks up, with all the great outdoor learning opportunities, and road trips to musuems.
The "almost" part of school year is another story, though. Little things, like the multiplication tables, or all those geometry formulas, that took so long to learn, can slip away over lemonade and popsicles. So, even though we continue learning throughout the summer months, summer slide can still be a concern.
So far, in our decade and a half of homeschooling, the easiest way to combat summer slide that I've found, is to keep the facts in view. Literally in view...
...taped up onto the walls around the house. This week, for instance, I've been busy taping multiplication stories next to C (age 9)'s bed (from Alan Walker's Memorize in Minutes, The Times Tables - reviewed here), where she can contemplate them before she drifts off at night, as well as an oversized map of the world, on the wall across the room.
This may seem pretty obvious, and might be something you already do on a regular basis - but really, don't underestimate the power of pictures on the walls. Something that you look at daily, even if you think you're not paying much attention to it, has the ability to fix itself permanently into your mind, as a memory. My older girls say they still see a flash of the pictures (from Times Tables the Fun Way, in their case) as they automatically answer multiplication problems, even now.
Changing it up for summer, with a review of this last year's basics - just the facts - can help to keep those little grey cells active without a lot of fuss or fun stealing bother.