1) I am sorry but I just accepted another offer that I feel is more suitable to my needs / more right for me./ more interesting to me.
2) I really appreciate your offer, but I must decline it.
3) Frankly speaking, I have been thinking that the position is not right for me after the interview.
4) I really think that position suits my education background better.
5）I don't want to accept a position that I don't think I will be happy in/offers me too little salary/ can't supply me with an apartment.
Your words should be professional and courteous. You may be turning down this position, but you might want to be considered for future opportunities. Savvy (有见识的) job seekers use every possible chance to thank each person with whom you interviewed and wish them and their company continued success.