Another tip I forgot to mention.
Anytime you call flight service when airborne always ask about TFR's, even if all you want is a weather update. If you talk to them airborne they will ask you if you are familiar with all TFR's and restrictions along your route. Too often my students would open their flight plan and then say yes to that question just to get off the radio. I would then always ask them if the understood what they just did, they just told flight service that they knew about any potential pop up TFR's that may have come up since departure.

Also, remember that when a forest fire pops up and they are using aircraft their will almost always be a TFR put up to help protect the fire bombers. I've only seen them up to 3,000 agl, but I can't say they are "always" that height.