A couple of thoughts about techniques mentioned earlier in the thread:
A few posts mentioned 'ground effect' at the bottom of the approach with a load; it's my experience that any ground effect with a slingload is negligable at best and I certainly wouldn't rely on getting any observable performance advantage from it.
A technique I used a lot when flying load to the tops of mountains, hot and high was to 'fly the load' using the mirror to a zero speed touchdown. You could be in a position that would prevent having sufficient power to hover but as soon as the load made contact with terra firma, you immediately gained extra performance. However, this required being very familiar with your aircraft and conditions and could easily end in tears if poorly judged. I'd only use this technique with netted, resilient loads!
Regarding the 'slingshot' technique, I've never had to do one. However, one dark night on goggles I was tasked to lift a very heavy piece of equipment from a boggy field in a hostile area. All the calculations were carefully done and it should have been well within the Wessex' capability. The load was hooked up and power gently increased to 3200lb (max tq) but it didn't budge. The cause, it occured to me at the time might be the 'surface tension' of the mud. So what I did was carefully ease the Wessex fore and aft whilst maintaining max power and the strop tightly tensioned. (I'd estimate the strop angle reached about 30 - 40 degs from the vertical). After a couple of cycles of rocking the load, one edge came free and the whole thing then came unstuck; it was a straightforward lift after that with a hover torque of about 3000lbs.
Anyone else tried similar techniques! J