having a bit of experience with metal detectors I can help you a bit.
when looking, find out if it searches straight down from the sides of the base plate (like a tube same size all the way down), as opposed to starting the same size as the plate, but moving into a cone the further into the ground it goes. Otherwise you could be missing a lot the further down you go, as the search field is reduced. Also find out if it needs to be ballanced to the ground it is being used on, if not you could get 'ghost hits' in ground with more metallic content.
Biggest thing that gets me, is watching people on the beach swipe it back and forth so quickly hoping to find that diamond in the rough, if your looking for something specific, your going to have to take your time. grid off an area and search specific areas thoroughly. Once you have a 'hit' center it by tracing a 4 leaf clover around it (move your detector away, circle around and come at it from another direction, stop once the hits repeats, do this like a clover until you have your spot) it will save time trying to figure out exactly where your hit is. We can usually pinpoint a thread of speaker wire to within a 6" circle before we even break ground to find it.
Swiping it back and forth is quick and easy, but you'll only find large objects, or hope to get lucky. Sweep slowly, back and forth, cover a 1.5m lane however long you want, than turn move left or right walk back and make sure you cover a 1/2 foot of your last lane to be sure you didn't miss anything. Or use a line of string if your going to be there for a while, so you know where you've searched and where your going.