So how about "that's okay, no hard feelings", or "wise girl/chap"(whatever), or "no, you're right.I enjoy working with you, so why complicate things? On paper they read a bit like Noel Coward out-takes).
Better still, you'll be seeing them during the day, rather than just propping up a bar with them in the evening. Work together during the day, pop out for a quick drink after work, Bob's your uncle. Everything is in order, from stress-busting massage at the desk to on-site counselling, so why not pick a mate there too? Hinting that you would like to take things further but never being specific can make the whole relationship uncomfortable, especially if it's a boss/employee situation.
This system obviously has advantages over night-time pick-ups in darkened clubs, holiday romances, (notoriously short-lived), or blind dates arranged by friends. High-powered City workers of the Nineties are accustomed to using the office as a complete life- support system. Making a pass is a dangerous manoeuvre in the workplace. Trying to blur the lines between business and pleasure will appear sleazy.
the excuse you present to your date for not seeing them again should be the same you give to everyone else. Be charming and never inflict unnecessary suffering.
If you are asked out by a co-worker and turn them down, keep that quiet too.
Do either of you have attachments - are you or this other person married/living with one another? Let them know it's a social thing too, or they may be unsure whether it's a meeting to discuss business.
Be light in tone but clear about your intentions (not all of them, of course).
Breaking-up without tears: If you find this person is not the man/woman of your dreams it would be cruel to keep things going beyond the first date.
Now, remember all those nasty little tricks people use to ditch someone will be absolutely no use in the workplace. The phrase "I'll call you" won't apply because you'll see their smiling face next morning in the office.
And at all costs avoid the following: "That's okay, no hard feelings - as I hope you won't have when I tell you your pay rise has just been cancelled." Or: "So Quentin in accounts was right after all, you aren't lesbian/gay".
Be polite, open, honest, and assertive, not furtive. And never be lewd (good, old-fashioned word) or make a pass.
Stationery cupboards: Or any cupboards come to that - and the more cramped the better.