Why are plants and insects uncommon as fossils?

"Because of the apparent frailty of their bodies, and the ability of
many of them to fly, insects are thought of being rarely found as
fossils. Any mention of insect fossils though, and most people think
of insects spectacularly fossilized in amber.1 However, insect fossils
have also been found preserved in fine-grained sedimentary strata,
including those associated with sequences of coal beds."

you can find more info at http://www.icr.org/pubs/imp/imp-329.htm

I don't think it is correct to say that either plants or insects are
rare as fossils. It is true that delicate structures don't preserve as
well as hard parts (butterfly wings vs. beetles for instance). There
is a rich history of both groups in the fossil record but they're not
considered as dramatic so don't often get the coverage that dinosaur
bones get.

Good examples of insects at
http://www.coloradomtn.edu/campus_rfl/staff_rfl/kohls/eocene.html

Some plant examples here
http://www.mnh.si.edu/museum/VirtualTour/Tour/First/FossilPlants/

jgriffiths link is to the Institute for Creation Research, not the
best source for information about Evolutionary Biology!!

If you count wood as a plant, petrified wood is quite common in many
areas.  More fragile parts of plants, such as leaves and flowers, are
less common.