Martinhoul, the way I see it a very large porcentage of out of the money options must expire worthless. The trick is to get something back, no matter how little, even with the in the money options.
What I did, when I was trading them ten years ago, was to buy them with a distant exercise date. That meant that the price went up and down with the share price.
However, (excuse me, I'm very rusty on this, now) probably about three weeks before the exercise date the time wastage starts to kick in. That is when the writers of options get the best price because it will come down to the share value all the way to a few days before exercise. Then, again, the option can be bought by speculators who may expect the price to go up or down in the few days before expiry because of results due, or any thing else.
I got disillusioned by vertical and calendar spread, straddles and all that. I ended up paying the brokers double or more, because each set of options you buy has it's broker's fee. So I I went with straight options.
It's slow, the way I did it and I changed to futures trading.