I use a non exspensive hair straitener at times.  I usually wash my hair the day/night before I plan to straiten my hair.
I am not sure but I assume that if I really got my hair wet or soaked it too would turn a bit curly.
Other wise mine is to not let your hair  be frizzy or anything.  it does well on getting my hair strait to the end.  depends on how you do it i suppose, but i like it.  i love how smooth it is and nice it looks.  so strait.
I had a perm along time ago.  about a year or so but it stills looks okay.