I currently have built a system that checks user IP, browser, and a random-string cookie to determine if he is an admin.

In the worst case, someone steals my cookie, uses the same browser I do, and masks his IP to appear as mine. Is there another layer of security I should add onto my script to make it more secure?

EDIT: To clarify: my website accepts absolutely NO input from users. I'm just designing a back-end admin panel to make it easier to update database entries.