I buy my F tickets as I am guaranteed a seat. I don't have the stress of worrying if I will be upgraded or tickets will be available when I want them. I can also go on whatever airline I want.

In Australia using a credit car incurs a 2% surcharge. So by not paying by credit card and earning points I save 2%. I also get a 'cash' price which saves. I figure I put some of that towards my airfares.

I myself have not found a formula for buying premium tickets cheaper on certain days. I find prices are higher on airline websites. I like to google First Class Specials or business specials and there are websites that lists all the offers. I'll then use that info when booking with my travel agent. (Once I've decided to go away, I don't want to do all that kind of stuff. It's like having an assistant do it!).