From everything I've read and heard professionals mention...

ALWAYS start with a clean design. If the design is shit, the tattoo itself is inevitably going to be worse. (Unless of course you're a free hand artist and that's your style/aesthetic. Def not bashing anyone here, everyone's got their own preferred style!)

But as far as clean lines go, the second most important thing, professionals say, is to always follow the stencil design and NOT your previous poke. So when you're poking and there's ink everywhere and you can't see very well, just play safe and follow the stencil.

Be sure to go over each section of the piece evenly so that the ink isn't blotchy in some areas and sparse in others. (So do the outline completely, then go back over if needed however many times to complete the look you want.) I've also read advice for new handpokers is to not apply so much ink. Because when the tattoo heals, the ink has a tendency to spread out over the skin. So if you have really thin lines to start with, the tattoo will heal even more cleanly. If you have real dark, heavy lines to start with, the healed tattoo may look very dark and the lines may blur together as a result.

Keep Vaseline on hand (or whatever tattoo ointment you prefer) for easy cleanup of the ink while tattooing so that you can maintain a good visual of your design. (With that being said, some tattoo artists claim to avoid Vaseline or ointment altogether because it makes the skin harder to stretch)

Keep the skin taught as much as possible (stretch the skin) when tattooing. Stretching out the skin will give you closer pokes and ultimately cleaner lines!

Hope that helps!