Evaluate all your options before you choose to decline an offer, make sure that your reason for declining an offer is compelling enough. Remember that the people involved in making you an offer have spent time and effort in extending one to you, therefore be polite and courteous. State the reason why came to this decision truthfully and professionally, a mismatch with the role offered, salary or even that you found some other role that is closer to your aspirations. Sharing the reason is helpful for the company, if there are too many declines the company can take some remedial actions. From your part, don"t leave the declining too late, that puts immense pressure on the entire bunch of hiring team who worked on your offer and sets the company back by a few months.