Does anyone know how I can fix it?

Put conditioner in after the shower and leave it in

The problem may be less a matter of specific straightening techniques than general hair care.

I don't know what to do about immediate fixes (besides sticking your head in the sink and going natural) so what I write won't have as much to do with your question, but I do have some long-term things to consider that will keep your hair looking lively.

First off, are you actually using a flat-iron? That's not a good idea, since it damages your cuticle, making your hair less shiny and predictably limp. Do like springdragonfly and go with a blow-dryer and a good hair serum for your straightening needs--better yet, use the wave God gave you. Your natural curl is usually the best thing for complimenting your facial features.

When I tell people this, the first thing they usually say, "I guess I'm not very usual then--my natural curl is horrendously frizzy/disgustingly tangled/insert equally repulsive description. " But another general thing about people who go straight is that they don't know how to take care of their hair to bring out the natural curl. Usually, they dry-brush, shampoo too often, or don't condition enough. Even if you continue to straighten--with a blow dryer and moisturizing serum, right? --following these tips (even the first to some extent) will help your non-naturally straight hair look better.

Dry brushing is never a good idea. It destroys good hair texture and frizzes it up. To avoid this, I usually stick my head in the sink under water as cold as I can stand, run some conditioner through it to detangle, rinse, and scrunch my hair. Of course, if you're planning on making your hair straight, then a better idea would be to use a serum instead, following its directions, pat dry with a towel, and blow-dry straight. Either way, your hair will look naturally shinier and healthier--because it will be!

Most people think they have to shampoo every day. This is usually a mistake. You should only shampoo when you feel like your hair is greasy. For me this is about every 36 hours, so I sometimes shampoo in the mornings. Another thing you should do is keep the shampooing action to the scalp--rinsing your hair keeps the ends clean enough.

However, if your hair isn't naturally straight, you should condition every time you bathe. I also use conditioner to detangle my hair instead of brushing, and all of this conditioning keeps my hair shiny and alive. Also, if you massage your scalp with conditioner, it keeps it as sweet-smelling as shampoo would.

In high school I used to straighten my hair, but I find it actually looks a little more alive since I learned to blow dry it straight instead. Takes a little bit of practice, but if you have a strong blow dryer and a big round brush and then try to imitate what they do in the salon (pin it up and blow dry sections starting from the lower hairs up) you can eventually learn to to a really good job. Some hairstylists are nice enough to give you a few tips as well. Good luck!

One thing I often do is when I'm straightening it, I try to rotate the iron in as I reach the ends of the hair, which gives the ends a nice round curl in towards my face. If you're already done straightening your hair, you can also do this by starting halfway down the hair and curling in as you straighten it again. Also, be sure that you are using some sort of flat iron straightening spray, because otherwise the straightening process is not very good for your hair and can scorch it over time. Hope this helps!