Most of how dark it goes is settled in the twelve hours after you leave, not by the paste.
People come back and ask why their henna faded in four days when their sister's lasted a fortnight, and it is almost never the paste. It is the first twelve hours.
Leave the paste on as long as you can. Two hours is the minimum, six is better, overnight is best and is what brides do. When it does come off, scrape it rather than washing it, and do not touch water for as long as you can manage after that. The stain keeps darkening for about two days after the paste is gone, and water in that window stops it where it is.
Keep it warm. Sitting in front of a fan for twenty minutes is for drying the surface, which is why we build that into the appointment, but after that warm hands take a darker stain than cold ones. A little oil rubbed in before you shower keeps the water off.
What not to do is anything that promises to force it. Lemon and sugar we do ourselves and that is the end of the list. Nothing else belongs on your skin and some of what gets recommended will take skin off.
And if somebody offers you a paste that is black in the cone and black on the skin in twenty minutes, that is not henna. Read the question about it on our questions page.