How much does your hair curl all depends on the cross-section of your hair. Some people have a circlular X-section and their hair are therefore straight without using straightener or anything. If your hair has a oval/ flat X-section, then it tends to curl.
I'm not a girl so i don't need to worry about it curling so much although it start to curls when it gets too long and thats when i need a haircut.
Try the Chi thing, it might work for you.
ps. Use conditioners and Pantene ProV is quite good, it has got VitB and the amino acid valine which helps straightening your hair. Also, keep your hair moiturized and the weight of your hair will keep it straight as well.

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