#1: Wait 3 months after starting a job to ask a coworker out. When you're new, all eyes are on you. So keep your mind on your work. If you're still smitten after the 3 months are up, do the 'evening check-in'. That means swing by their cubicle for a chat on your way out the door. If it goes well, make it a habit. Then one night you can say, "Hey, let's get out of here and grab a bite." See, nice and casual.
#2: Don't bring your personal life to work. Whether you're quarreling or cuddling, leave it outside.
And a final tip when it comes to dating a coworker: If the relationship should go south, break-up on a Friday night.
That way you'll have the weekend to recover and you won't make a scene at the office.
If you'd like to go further, check out "The Office Romance" by Dennis Powers.