I'm talking mainly domestic here - especially Qantas... international flights use a similar pattern, but do have bigger movements due to sales etc. Virgin and Jetstar do similar as well, but I don't follow their pricing structure as closely as Qantas.
If you know where you're going, then normally in advance as far ahead as possible is good. That will get you a standard priced fare in the discount category.
Otherwise, wait until the sales happen, and jump in then. This will normally get you an even better fare.
Even though we have competition now from 3 domestic carriers, the prices are still pretty similar. Sales do happen, but you can't really predict when they will happen - or that they'll cover the dates you want to fly. Also the discount fares tend to be limited so you have to know what you want and book it quickly.
You don't see too much price movement happen esp. in Aust domestic flights. The airlines tend to keep prices pretty similar. Obviously once they sell a certain number of tickets on a flight they've made their money and don't care about the rest of the seats.
The issue with discounting by allowing standby is that it gets people used to rocking up to the airport for a cheap flight. The same people who would buy a higher priced trip off the airlines website now gets a cheaper one... so in fact the airlines lose profit... (Hint, if you have to fly, you're gonna go no matter what the cost is in most cases)
Some examples:
I want to fly Qantas from Syd - Mel
Thursday 31 March:
Most cheap seats are sold. There is one flight @ $69 + tax, and another 2 or 3 at $83 + tax. The average price is $107 - $126 for super saver tickets with some going for the late $100's.
Fully flexible is $327+tax, Business is $521 + tax.
Now, instead, change the date to Thu 21 June.
There is a lot of availability for $69+tax, plus somes seats at $83+tax.
Super saver is mostly at $107, with some at $126, and some peak flights ( ~6pm) around $200.
Fully Flex and Business are the same prices as above.
So, you can see that the prices are relatively the same -- but that there are more discount tickets available if you book earlier.
If I took the same flight tommorrow (This is a bad example .. due to Easter Monday flight traffic meaning most flights are pretty full), there are no red-e-deal tickets available, but still super savers starting at $107 with most around $150 or $180 + tax. These prices won't go down before the flights happen...
Sometimes you will see some last minute sales ... but that only seems to happen when a flight is pretty empty and they need to sell seats. Don't count on this happening too often -- Qantas especially seems to have their yield management on domestic flights well controlled and rarely need to do this.