I can tip most of our sheep, but we have a couple of massive old ewes who are marginal, and the tup, who I have never tipped yet (I now just walk up to him whilst he's lying cudding, and sit on top of him before he has a chance to work out what's going on!
).
With the big ewes, I've found that surprise makes a big difference. If I can turn their heads before they've had chance to realise what's happened, they don't stiffen up, and tend to just collapse in a heap. If the first attempt fails, there's no point in trying again, because they will tense up in advance, and I'll have no chance.
The same goes for one particular ewe, who has learned that she can avoid being tipped by dropping to her knees. That makes her basically impossible to tip, because she is then heavier than me, more stable, and with a lower centre of gravity. The next time she tries it, I'm going to try tickling her, to see if she rolls over like a dog!