In my experiences with Salvia there is a recurring moment early in the experience. The "Oh, right, this part again" bit. I call it "the swamp", and for some people they just fly over it or take another route altogether and just blast right off. I always, always have a strong thought in there along the lines of "WHY did I put myself here again? Surely other people don't go through this?" For me there's a stage which is almost exactly the same every time, a point where I begin to hive and split into a multitude, and all of my senses and thoughts and memories are cross-wired. The walls are memories, my body is splitting into a 2d bristling fractal but I'm also my car from 1988, voices and prickling, bristling, hot hiving somatic dismantling. It's like I'm experiencing somehow every single piece of my ego, all of my memories and psyche and understandings swirling amongst my cells - I have a problem inhabiting the "experiencer's seat", which is the true nature of ego death - leaving behind that hellish somatic swamp in favor of a higher seat in which you can "watch" it from above by letting go of it all - leaving it all below.
While I still haven't gotten that particular part down, being in the swamp was something I did enough times that in a way I got used to it. It was never pleasant for me, but essentially there was that familiarity: "right, this part." From there there's little you can really do aside from laying back and let it do what it will, and for me this rarely means breakthrough. Out of 15 or so times smoking, I've only had 2 breakthroughs. One I just forced myself through by taking more-more-more. The other one followed a full day of being relaxed and taking time to myself to just be, and I meditated going into Salvia country for an hour. There was a frightening moment when I realized, "I don't want to be here - I want to go back" but I didn't know what "back" was or what I used to be. I had no memory of smoking salvia or even being a human. All I knew was that, at some point, somehow coming into this new form, I changed, and a door shut behind me, and now I'm in. This was all I needed - it was perfect for me, in fact. I immediately let go and was washed into a deep cosmic stream.
Be prepared to just ACCEPT. That's my biggest advice for having a positive experience - and there is definitely tremendous positivity available with this plant. Accept whatever happens and welcome it. It may feel like YOU are being brought into ITS realm but also remember that what you're exploring is also your own mind's experience. Bring a deep state of warm acceptance to whatever the plant has to show you, however strange or somehow traumatic it might be. It's okay to be nervous. Once the trip reaches a certain point that won't matter anymore - you're in a new space which demands your attention.
I wish you the best of luck in your journeys. I look forward to hearing back about what you discover!