If you are using a detector on private land/farmland you should seek permission from the owner. If you don't you could find yourself charged with trespass.
One of the lads in work is an avid detector and he usually strikes a deal with the owner of a small share of anything he finds. I think he makes a first offer of 5% when he sells a find but sometimes has to go higher.
he has found roman coins, belt buckles, bracelets and various other bits and pieces. 4 years ago he found a Roman coin which sold for over £18,000. Luckily for him he had a written note from the landowner he could search for free so kept the whole lot and bought himself a brand new Volvo car.
One of the tips he gave me was to look at any inland ground which had remnants of sea shells. This usually indicated settlements from long ago and may, just may, have something worth finding.