As you can see in PoD, our rivers aren't huge and the challenge lies in getting the fly down as fast as possible to "fishing depth" and then keeping there. If the fly sinks too slowly, you'll quickly loose half of the swing and not fish the correct depth on the far bank (and that's where all the fish are - we all know that).
Look at the picture again, and you'll also notice that there is plenty of flora in our rivers, and frequent snags are part of the game. Choosing the right sinkrate makes a big difference. You'll want to fish deep, and it's fine if you touch/catch weed every now and then, but too often gets tiresome as you have to retriece the fly, clear it of weeds and re-cast. Not exactly effective.
So - if I come to a (maybe short) stretch of river with less depth, do i change sink rates. Well, yes, sometimes I do, but most often, I simply change my fishing depth by speeding up the swing just a little.
It happens a lot - the water maybe slows down because the river widens a little and I start getting too many snags. Rather than putting on a less dense line, I speed up the swing using too different techniques. Normally, I cast and the take 3-4 steps downstream to let the fly sink at the opposite bank before I tighten up and start the swing. Simply by changing that order (take 3-4 steps and then cast, allowing the swing to begin almost immediately), I can lift the fly a little. Sometimes I might adjust a little by letting a little line go to gian a little more depth.
The other method I use quite often is a simple figure-of-eight retrieve, and if that isn'y enough, I'll use that in conjunction with the above or even do slow strips.
Something else you need to learn is when to strip back the shooting line for a new cast to avoid snagging in the weeds along your own bank. If you wait too long, you'll snag almost every cast, and while it's certainly worth while to do what you can to fish your own bank (preferably by stripping back the fly 2-4 meters if possible), it's also really tiresome to catch weed every time you setup for a new cast. My rule of thumb is this: As the back of the shooting head comes to my own bank, the fly is still pretty far from the bank - a straight line between fly and back of shooting head might exhibit an angle of 30 degrees or more).
Then it's a simple matter of timing when to start stripping, so you can lift to a new cast before the fly gets too close to the weeds on your own bank. That of course changes a lot with current speed, fishing depth, legnth of cast etc etc. Just remember: If you snag on every cast, do something else :-).
Have a great weekend!