I say sorry and then tell them some reason, which doesn't have to be a real one. For example, I'm sorry, I have lots of things to do. Or I don't answer clearly like, I'll consider or I'll call you later. But actually, there was few circumstances that I had to reject something politely.

Hello, Seong Eun!

Thanks for writing your homework.

Have a great day! See you on our next class. ^^

-T.Jhozel 

I say sorry and then tell them some reason, which doesn't have to be a real one. 

>> Correct!

OR: I apologize and then explain why, which does not have to be a real reason. 

For example, I'm sorry, I have lots of things to do.

>> Correct! 

OR: For example, I'm sorry, but I have a lot of things to do.

Or I don't answer clearly like, I'll consider or I'll call you later. 

>> Correct!

OR: Or I don't respond clearly, such as I'll think about it or I'll call you back later. 

But actually, there was few circumstances that I had to reject something politely.

>> Correct!

OR: However, there were only a few situations in which I had to politely decline something.