In the early season rising fish can often be in short supply.  I always tell my guiding clients that the No1 skill in chalkstream fly fishing is not fancy casting or fly selection it is observation and stealth.  Don’t thrash the water aimlessly.  Watch the water constantly.  Scan the water in all directions looking for rises.  A ‘oncer’ is a fish that you see rise just once.  On a day (or a part of the day) when there are  few flies hatching you must notice these fish and act on them.  Most Trout will stay put in their ‘home’ lie and the fact that the fish rose tells you that it is alert and looking for surface food.  Try to note exactly where the fish rose.  Make a note of a bankside feature – perhaps a piece of bankside vegitation, a small bush or a bunch of reeds.  Wait for a few minutes to see if it rises again.  If it does you can mark the spot more precisely.  If it does not you still have a good idea where the fish is likely to be.   Think what the Trout might have risen to take and pick a fly that is a good representation of that food item.  Approach cautiously to within casting range.  Don’t be in a hurry.  Try to make the first cast count.  I often aim to put a cast just short so as to judge the distance better for the main cast.  Ideally cast about 6″ or a foot to the side of the fish and land it about a yard above the fish.  If it doesn’t take it first cast it is worth a few more tries.  If after half a dozen good drifts over the lie the fish has not shown then either try a different fly or move on to another spot.  But its amazing how often spotting a ‘oncer’ results in a good take in the first three casts over the right spot.