I'd worry about getting it to work first of all - I imagine your first dozen or so attempts won't work anyway.
If you found ayahuasca comfortable I can't see smoked DMT being a problem. People say all sorts of daft shit about ayahuasca too, but I've always found DMT a very friendly psychedelic.
Why would it take a dozen attempts to make it work? I broke through my first time smoking DMT. Seems like if the dose is right, you smoke the material right and inhale fully and hold it in it has to work. I'd say 30 or 40 mg, smoked without weed (I hate mixing DMT and weed, even though I'm a pothead) all in one hit and held in as long as possible and you'll probably break through.
I love ayahuasca but find smoked DMT to be absolutely fucking terrifying, reliably. I've always found ayahuasca to be smooth and comfortable- clear headed, beautiful and natural feeling, like an extension of normal consciousness in a way. I've always found DMT to be a very friendly psychedelic in any ROA except IV or smoked. Smoked DMT is very, very harsh to me though.
DMT, as soon as I smoke it I get washed with overwhelming waves of fear and anxiety, I'm experienced (probably 50-150 smoked DMT trips) and clear headed enough to look at the fear as what it is (drug induced nonsense), but I still feel horribly uncomfortable for the duration of the trip.
Smoked DMT is too much, too fast for my tastes. It's so quick that I take very little away from it and hits so hard and fast that I can't avoid the anxiety. Plus it just makes me feel bad.
So for some people, ayahuasca= no problem and smoked DMT= terror.
Anyway, I personally don't know of any way to prepare for DMT. Reality will shift entirely, just be ready for one hell of a strong trip. If you feel bad during the trip, just think to yourself "It's only a drug making me feel bad, it'll be over in 5 minutes or less" and allow the drug to take you. Resistance is futile.
I don't think salvia will really prepare you, I find the onset of salvia just as sudden and terrifying as the onset of DMT. But YMMV. Most people consider salvia much more unpleasant than DMT.
DMT is really, really, really cool shit, and imo it's not as hard to handle as people say it is. The main problem is that smoking DMT hits you very, very fast and very, very hard. So just be ready to be as high or higher than you've ever been within a few seconds of exhaling.