There isn't really a trick involved - it is just a matter of changing where you place your increases each round. This will work for any basic crochet hat that uses the same stitch all the way through (no cables or clusters or shells...follow the pattern with those). All hats begin the same way - with a ring that you crochet so many stitches into (lets say 8). Then, for so many rounds you must increase stitches to get the hat bigger. Usually the increase in each round is equal to the number you first started with, in this case 8. So each round after the first you would increase 8 stitches at even intervals. Patterns are usually written so you do the increases like this:
Round 1: SC 8 into ring
Round 2: 2 SC in each stitch around
Round 3: (2 SC in first stitch, 1 SC in next) repeat around
Round 4: (2 SC in first stitch, 1 SC in next 2) repeat around
Round 5: (2 SC in first stitch, 1 SC in next 3) repeat around
And on and on like this until the increase rounds are done.
Increasing this way will cause corners where the increases are and you will see lines coming out from the center of the hat. To avoid this, instead of doing Round 4 like (1,2), 3, 4, (1,2), 3, 4... do 1, (2,3), 4, 1, (2,3), 4 ... and Round 5 like 1, 2, 3, (4,5)... and Round 6 like 1, 2, (3,4), 5, 6... and Round 7 like (1,2), 3, 4, 5, 6, 7... Still keep the increases evenly spaced throughout the round, but change it so they never line up over the increase from the previous round. This will give you a circle shape with no corners or lines.
Here is an example from my Timeless Hat pattern:
The square on the left was crocheted as written in the pattern (see the lines that make an "X"?), and the circle on the right was crocheted by staggering the increases (no corners, no lines). This example is a little extreme because most hats would make closer to an octagon shape, but the way the increases are in this particular pattern, you get a square. And if you worked it up that way, it would be fine. It would still be a circular hat at the end, but you would possibly see 4 corners sticking out of the top of your head that I'm not sure would mellow out after washing.
Round 1: SC 8 into ring
Round 2: 2 SC in each stitch around
Round 3: (2 SC in first stitch, 1 SC in next) repeat around
Round 4: (2 SC in first stitch, 1 SC in next 2) repeat around
Round 5: (2 SC in first stitch, 1 SC in next 3) repeat around
And on and on like this until the increase rounds are done.
Increasing this way will cause corners where the increases are and you will see lines coming out from the center of the hat. To avoid this, instead of doing Round 4 like (1,2), 3, 4, (1,2), 3, 4... do 1, (2,3), 4, 1, (2,3), 4 ... and Round 5 like 1, 2, 3, (4,5)... and Round 6 like 1, 2, (3,4), 5, 6... and Round 7 like (1,2), 3, 4, 5, 6, 7... Still keep the increases evenly spaced throughout the round, but change it so they never line up over the increase from the previous round. This will give you a circle shape with no corners or lines.
Here is an example from my Timeless Hat pattern:
The square on the left was crocheted as written in the pattern (see the lines that make an "X"?), and the circle on the right was crocheted by staggering the increases (no corners, no lines). This example is a little extreme because most hats would make closer to an octagon shape, but the way the increases are in this particular pattern, you get a square. And if you worked it up that way, it would be fine. It would still be a circular hat at the end, but you would possibly see 4 corners sticking out of the top of your head that I'm not sure would mellow out after washing.