Depends on what function / age group i'm playing to.
For an older crowd or a more formal function, I would usually start with something that isn't too old, but that most middle aged - older people will remember and can dance to. Diana Ross - Chain Reaction is a good example. Not too heavy or dancy to start them off.
Wedding Reception - I'd start off with the Bride & Grooms first dance (their choice - which is usually but not always! a smoochy) followed by something slightly faster but still easy listening. Then i'd go into something cheesy that everybody can identify with - Kool and the Gang - Celebration - predictable but does wonders on the dancefloor.
Other good dancefloor fillers - to encourage them to start dancing include, Billy Joel - Uptown Girl, Dexy's Midnight Runners - Come on Eileen, 4 Seasons - December '63 (Oh what a night), Billy Ocean - Love really hurts without you, Edwin Starr - Contact, Tom Jones - Sex Bomb
Judge it by the function and age group and trust your instincts.
For less formal mixed Teens to 30's age groups - modern classics like Rock DJ, Shania Twain - Man I feel like a Woman etc still work quite well.
Don't forget to use Kazz's tip at Family Functions, where there may be a lot of kids in attendance. Use the first 30 mins to 1 hour to play to them (No really heavy stuff tho like Eminem!!!) if they are younger kids then do some Games for them as well - it impresses the adults that you are truely catering to all ages, and keeping the kids from under their feet whilst they have a natter. It also impresses the client since you are working from the word go, and not sat at the bar just playing background music.
To finish off - again - age group and audience driven by then you should have a good idea of what will work

. Often I start lowering the pace by playing some Classic Motown, then going into UB40 - Red Red Wine, Rufus & Chaka Khan - Ain't no body. The as a final track Either a smoochy or New York, New York or You'll Never Walk alone is predictable and hardly original but hey, it works!.