I try to find resin planters with thicker sides. They might cost a few dollars more, but that extra bit of insulation really helps sunny planters during the heat of summer.
Here's a picture that shows what I mean-
http://www.homedepot.com/p/Gro...
You can also save $$ by using inexpensive plastic refuse containers for your shadier/cooler areas.
http://garden.org/ideas/view/c... . They're readily available in many shapes, sizes and colors.
