下さい is used for asking someone to give you something. ex) お水を下さい。May I have some water?
ください is used for letting someone to do something. ex) お掛けください。 Please have a seat.

As Garret says, however, I think many Japanese people (especially younger generation) don't know the difference between them, and they don't care whichever words are used.

I am learning English, and I am trying to explain it easily. I hope you can understand it.

ありがとうございます、まい。

Sorry I misunderstood your original post. I didn't see the heading :P haha 

ください is what my friends say. But it could just be preference. Sometimes people choose to use or not use kanji, even though both are ok. Often girls use hiragana more (from what my friends say) because they think it looks cuter. Haha 

i don't know if that helps, I'm still basidslly a beginner but that's just my obserration

Thanks Garret for your input. I am looking for someone who is a native who can answer this question so we both can make sure about this.

and I suck at typing on my iPad I guess haha