passionflower can increase the strength of a trip to beyond the normal maximal level, WITHOUT all the side-effects that occur at higher doses. It makes the visuals a little more colourfull, more 3dimensional and they become more overwhelming; they completely encompass you. You get more body-high effects like a sometimes mild sensation of floating and you'l feel more relaxed and peacefull. It also lasts longer. Up to 18 hours.
This seems to be the best way to get extreme LSD experiences.
Oral cannabis is VERY synergistic; it seems as if the side effects of both cancel eachother our; you don't get the speedyness of LSD that much and you don't get the fuzzyness of cannabis. It's extremely euphoric, you only need to take enough cannabis since the LSD tends to overrule the cannabis at low doses. You can get the same effects that you would normally have at the saturation-point with half the amount of acid, AND you'l get more euphoria.
Datura is very good as well, but only safe in low doses. It makes the LSD warmer and the visuals more vivid and alive and it may give you strange and pleasant tingling sensations.
Fly-agaric potentiates the visual effects dramatically, as well and has a stage of mental sharpness first, followed by a pleasant sort of drowsyness.
With the first three mentioned, you really get a much more pleasant trip with more feelings of euphoria, peacefulness or wellbeing, and you can take the trip to a level beyond the saturation point.
Alcohol diminishes the psychedelic effects somewhat, is my experience, although it's not an unpleasant combination. LSD temporarily increases your tolerance for booze somewhat, like many stimulants do.
I also believe that physical activity, prior to taking psychedelics, generally has a positive effect, but i have never realy established as a fact if this always works with LSD. I believe it does.