Truth is, some girls just aren't into clothing and product like others. Some just don't see the point and consider it being fake, some prefer to be comfortable, some just have low self esteem and some think they just look that good that they don't have to put forth more effort.
Personally, I love my jeans and t-shirts, but I also love my hair and make up. I dress up very well every time I go out, though. I didn't used to. My ex was pretty much a dick and wrecked my self image. After that split, it took me a while to become more comfortable with myself again and I took baby steps.
This might take a couple of months to pull off, but I would suggest trying this route:

1) Don't buy her something that is a far departure of what she normally wears. She will see this as judgement. It might not be a pretty result.

2) Instead, try to follow along her current style, just vamp it up a bit. If she like jeans and t-shirts, try to get her in a pair of jeans that are dressier. The trick is to get her more comfortable in her own skin.
These might work:

http://shop.guess.com/Catalog/View/Wome ... B0153D3100 .
Doesn't have to be the exact same ones, but look at the color and cut.

http://www.victoriassecret.com/ss/Satel ... vsdWrapper

3) When you get her into something more form fitting and showing just a little more skin, give COMPLIMENTS!!! You have to give a little to get a little, right? You have to let her know she looks great or she won't keep progressing.

4) Go a little more bold, such as:

http://www.victoriassecret.com/ss/Satel ... vsdWrapper

5) Hmmmm... more compliments! Also try suggesting espadrilles to go with them. These will be a little more comfortable because they give more to balance on and you can get them in lower height. Remember, girls aren't born knowing how to walk in this stuff, we have to learn and it isn't easy. They hurt more the higher they are.

6) Offer her a trip to the salon. Girl's don't like shelling out $85 + for that and it takes a lot of time to do at home. If you offer, she'll most likely take you up on it.

Her confidence will be higher, making her want to wear these things. Don't load these on her all at once! She will be defensive. Try one a week or so. She will get bolder on her own if you play your cards right. If none of this works, you've got yourself a woman that doesn't care about being a woman. Either accept that she just wants to be comfortable, or it's time to move on.

Good luck!

one thing bothers me. She doesn't dress in the way i like girls to dress. That is: tight pants, thongs, dresses, high heels etc. I really like girls to dress a little bit 'bitchy', to say. I get really aroused by it.

This girl on the other hand sticks to simple t-shirts. The issue goes so far that sometimes i'm looking to other, more 'bitchy' dressed girls, and start to fantasize about them.

Go find a girl that is EXACTLY what you want and stop wasting this girls time.

Relationship FAIL. This is doomed before it even starts.

I hate being harsh, but seriously dude, read what you just wrote. Healthy relationships are about accpeting the person exactly the way they are and sharing part of your life with them, even if that part is only a little while. The second you start putting expectations on the other person or whishing they did things different, what you are really saying to your self is "I want a DIFFERENT person".

This girl isn't going to change, even if she dresses bitchier or whatever, she is going to resent you for it because she knows she is only doing it for you and not for her. Plus watching you check out other girls that AREN'T like her will constantly slap her with the fact that YOU LIKE OTHER GIRLS THAT AREN'T LIKE HER.

If you are going to date this girl, like her for her - lame t-shirts and all. OR be realistic with yourself and realize you are settling. Go find a girl that is EXACTLY what you want and stop wasting this girls time.

also my ex'es had issues. One acted too much as a bitch, the other never had to say something interesting, etc. All the time there are issues. Now its the clothing, and in a way i'm happy it is ONLY the clothing, since in the end you can try to compromise on it