I suspect there is as much art to science in this and that you use any trick going depending on type of structure, local geology, the season and the climate. low angle is certainly one, but winter and summer sunrise/set are 90 degrees apart so a different view is possible. differential melting of the morning frost for just a short window sounds such a trick too. but also important are parch and crop marks; differential growth of a crop to show ploughed out ditches which will occur at some variable time according to the crop in question. in drought summers like 1976 and more recent years then dry and moist areas of grasslands show parch marks clearly (good for stone walls). I have a feeling that the Dorset Cursus most of which is invisible from the ground can best be seen from the air in the fresh plough soil with a chalky stripe for the bank and a dark stripe for the ditch against a speckled background.
other things just need an aeroplane whatever, such as the Nasca desert figures and Masada Roman forts to see them nicely. (actually a cable car does nicely for the later…) see http://ourpasthistory.com/Gallery/album90
good for a big laugh as it’s a ‘hairy’ experience… 
