From Day One I've always switched the machine on the moment I'm leaving the jalopy (no matter how far I have to walk to my intended field) and kept it switched on until getting back. This has reaped dividends, the best of which have been two denarii:
The first was on a trek which involved crossing an "unproductive" area of a field in order to get to some pasture. With only desultory swinging involved, a "dig this, sunshine" signal came out of nowhere and a lovely little Antoninus Pius was the result. Subsequent gridding of the area provided nothing of any note and it seems this little fellah was all on his own. I got lucky, I suppose.
The second was just before the entrance to some pasture I intended to larrap where this Septimius Severus yelled at me. Again, nothing else even remotely interesting has been found in the vicinity. I got lucky, I suppose.
Then the most bizarre circumstances led to the discovery of a lovely medieval strap end front plate. The field had been mostly disked but in order to get to the flat bit I had to do my Edmund Hillary impersonation and cross a strip of ankle-breaking rough plough. At one point I lost balance and, with arms and legs all over the place in order to remain upright, I got a signal from the flailing machine. Once back on me pins I had a skank about and found the item whose signal had registered through the panic. I got lucky, I suppose.
I've had a few other bits and pieces under similar circumstances (the machine being permanently switched on I mean, not falling base over apex) such as a crotal bell and book fitting - enough, anyway, to cause me always to have the machine switched on. Did I get lucky with the above finds? Of course I did. But then every find can be considered fortunate in that you're lucky enough to walk over it in the first place. The trick, I think, is to maximise that luck by whatever means possible and having your machine switched on is a decent start
I guess having thd machine on certainly helps 
