The video below is a discussion about how to fly cross country and make a sensible field selection and landing. It is intended to be watched in conjunction with the other materials linked to on this page.
A really good resource for field landing training is the BGA field landing videos, which are linked to below.
Introduction; The process of field selection; Size, slope, surface
Obstacles on the approach; The effect of wind; The biggest is not always the best!
Slope: identifying and dealing with it...
Surface, early spring to mid-season; What's growing, can we land on it?; Winter and spring cereal crops; Tram-lines and tractor wheelings; Oil-seed rap; Introducing grass fields.
Grass for silage, hay and pasture
Horses, stud farms and gallops; Sheep; Cattle and stock-fences; Identifying fences and avoiding landing across colour changes
Overhead wires; Spotting the poles; Approaching harvest
Harvest time, stubble and straw; Random tractor tracks; Land along the direction of cut; After the harvest; Happy landings
Additionally, this website has images of a variety of different types of crops. It's well worth studying.
The BGA has published this safety briefing on field landing which is a good summary.