Question about flat irons?

Okay well let me start of by explaining my hair, i have natural blonde hair that is thin enough but not extremely thin. I damaged it quite a lot a few months back but its back to shape now. Okay whenever i straighten my hair, after about a half an hour it starts curling, going frizzy and poofy and it looks terrible. Ive tried ghd's and they dont work at all. Can any one recommend a good flat iron that will not only straighten my hair but keep it straight and smooth for the whole day ? ( nothing too expensive please ) Also if you know of any other ways that can help keep it straight when i do straighten it please let me know :)