Proper response to 不好意思
没事 and 没关系 are good responses to 不好意思. You can also use 不要紧 and 没什么.

On 搜狐问答 they say that in normal circumstances the difference between these words is really small, but there can be a difference in meaning:

没事 is rather used when there are no losses and no harm. 不要紧 is rather used when the losses or harm is small and 没关系 when the losses and harm is big, but you don't want to blame and the other person doesn't need to take responsibility.

On Nciku they say that 不要紧, 没关系, 没什么 and 没事儿 all mean "to be of no importance". They can also be used as a response to another person's apology. Furthermore 没事儿 can mean "nothing serious". If 不要紧 is used in a subordinate clause, there is harm involved and the harm is mentioned in the main clause. On Nciku you can find some examples.