Look carefully at the picture above. Can you see where each round starts and begins? If you can, then you've really honed your crochet eye and should be proud! It's barely noticeable, and I've finally found the best and easiest technique for joining rounds. If you've never worked in the round before, see my other tutorial first, then come back here.
Once you reach the end of your round, locate the top of the beginning chain of the round. In this case, I chained 2 to start the round and the arrow points to the space where the chain 2 connects to the first double crochet. You're going to insert your hook through the TOP TWO strands of that space.
Here's another view, with the hook actually inserted under the two strands. Yarn over, and pull that loop through the two chains.
Et voila! You have now beautifully completed the round and won't have problems with a gap or bump where each round ends.