Extending downwind is a bad habit, if it is any further than the point appropriate to a flapless approach and often leads to absurdly huge circuits.
Going round from base leg means crossing to the deadside, then fitting into the circuit as required. Other traffic joining must give then way.
Straight in joins should only be flown if there is no other traffic - or on an instrument approach, of course. If there's any other traffic, then join on the deadside and fit in as necessary.
I was once duty pilot at Benson supervising solo students bouncing Bulldogs off the runway. Whereupon a plummy-voiced corgi-carrier in a 146 announced he was joining straight in, having dropped off the royal at some venue or other.
I directed ATC to tell him to join on the deadside as there were 3 others in. He queried this, so I told them to repeat it. But he did as instructed and fitted into the circuit without difficulty. Later in the bar he queried this with the CFI, who backed me up and advised him to read the Flying Order Book.
http://www.caa.co.uk/docs/33/ga_srgw...osterJan09.pdf provides a good depiction of the standard overhead join.