Most things are hard to see from a search aircraft. As you say the flash is the best. The way I have always heard the tyre burning thing referred to has been as a smoke signal to be seen from the ground. In the more remote regions of Austalia a smoke signal from a tyre is the best way to be spotted by someone on the ground. The best times to burn is at dawn or just before dusk. This is when Cockies (farmers/station owners) come out to look at their property. If they see any smoke at all you will have a visitor real soon!

I have heard it recommended that you should burn the tubes first and then the tyres after all the tubes are gone, that will give you five days of fires. Over here in the outback tubes are not uncommon because punctures are very common.

However the best way to stay found over here is to get a HT radio and join the VKS network. If you have trouble just radio and help will be on the way.