One thing that I find helps is to make sure I have a fiber's length plus half (also known as 1-1/2 times the "staple length") of loose fiber on the end of the spun part, and join it to a fiber's length plus half again of pre-drafted fiber on my roving. Then, with that 'doubled' overlapped part, continue drafting as per normal to stretch it out to normal width.
If the spun part ends abruptly or is very thin, I will make sure to fluff it out, and even move back up to a thicker part to ensure there is enough fiber for a good join.
Also, I hold the joining part of overlapping fibers when I first start putting twist in, waiting to feel the twist between my fingers before moving back to uncover more unspun fiber.
If I really pay attention and don't try to take it too fast, that usually works for me and is usually indistinguishable from the non-joined parts.
(based on a posting by me on spindlers, 12/17/2001)
(drawing is (c) The Bellwether, from Spindling: The Basics. Used with permission)