Clarke,
4x4's for the main posts
2x4's along the side to tie all 4x4's together
2x8 across the top to complete the slider & seal everything together
Landscape / lawn bender board for the top ( cheap / easy material to use )
* for side braces 2x4's as Rod said. Also depending how tall the slider is we used 8 foot landscape / tree stakes & drove them in 5 + feet to provide extra support to the 4x4's - its solid as a rock.
Here's the basics.