I've been throwing sidearm since I was a kid, and it's still my goto throw for distance. With open space to work with, 400'+ is possible with pretty high consistency, especially if you make intelligent use of whatever wind is available.
For pure maximum distance, I agree that it's hard to outdrive a destroyer. I keep two Star plastic, 167-169 Destroyers in my bag for max-D shots (forehand and backhand). However, I throw sidearm with every disc in my bag -- mids, putters, all drivers, etc. With practice, you should be able to flick your putter flat 200-250' with pretty high consistency.
The caveat to the Destroyer is that it is squirrellier than a lot of other discs, so it's hard to know where your max-D shot will land. For consistency, something a notch slower -- like a wraith, orc or valkyrie -- is preferably for most shots.
Most people use too much arm or roll their wrist as they throw, and this produces a tendency to flip the disc over early or burn it into the ground. Other common practices are to follow through too high and the disc stalls out nose up.
Keep a nice, flat throw, elbow close to the body, hard snap -- similar to the motion of popping a towel at someone -- and just get some field practice working on different lines.
My favorite disc for flick shots is a Champion/CE/Proline Firebird, somewhere in the 160s. Both the F (FX) and FL molds work very well. The Firebird is incredibly consistent and the spike-flick shot is unbelievably predictable for shots inside 250', and the low, skip-flick is unbeatable for getting around corners, as long as the ground is accommodating to the skip.
If I was recommending a practice regimen, I'd suggest playing several rounds of golf where you keep two simultaneous scores on every hole -- one for your backhand and one for your forehand. Place duplicates of every disc in your bag and throw the same disc with each hand throughout the hole. For putts, use this as a chance to practice flick putts and Turbo putts, since you'll need both shots someday in the woods. Keep separate FH/BH scores and see how they compare at the end of the round.
I started by throwing all flicks, so playing with two scores is how I learned to throw a BH -- it forced a lot of shots I would have never tried otherwise, and was of immense help in improving my overall game.