Today I ran across a tutorial which is supposed to reduce that diagonal join and give a straight join. I found the link in a Happily Hooked magazine blog post. I know of the magazine but I do not subscribe. It is just shy of $40/year.They have a great blog which provides interesting and helpful information.
You can find the tutorial to eliminate the diagonal join (with hdc) at Charmed by Ewe. Kudos to her for coming up with this as I know this has been a problem others have fought with too.
I also found another blog through the same post. It is about crocheting in the round with single crochet and getting an invisible seam. In the first part of the video she shows how others might work single crochet joins incorrectly - she tells you this is the incorrect way to do it. If you follow your patterns correctly, you should already not be doing this.Most patterns will explicitly say to work the first sc in the same stitch. It's good to see how one could make this error, however, if you're not paying close attention.
She then demonstrates the correct way to join single crochet in the round. This is, as she says, part one to getting a good seam. The important part of the tutorial is how to get the seam invisible. The post is at Crochet Ever After.
Those tutorials are for working sc and hdc. I might also add that when I work a hat in the round using double crochet, I like to join with a slip stitch, ch 1, and work my first dc in same stitch. If you working an increase here, you would work ch 1, 2 dc in same stitch. By using a chain 1 instead of the usual ch 3, on my next round I can join right into an actual stitch, rather than a chain. I've been doing this for years now. It is a tip I read when working a pattern for a hat over at The Gingerbread Lady. I have made her Big Button Beret twice. I love the look and the pattern.
Here are pictures of my dc join.
And just for fun pictures of one of the Big Button Berets I made. I did not add the button. Of all the hats I've made, this is one of my favorites even though I used thin yarn and it took a while to complete. You can read the blog post I did when making it here.