I've always had this theory that the plainer one is, the more of an effort one should make. I'm sure it's not entirely PC or moral for that matter but it's probably an evolutionary trick (because if only beautiful people got to pair up and mate then by now we'd all be gods and goddesses!).
Example 1: crossing the road. I don't know about you, but I always wait to make sure that car (a) has seen me, and (b) is slowing down. And then I always give a friendly wave as I walk across, just in case.
Example 2: being grateful for the attention. If asked on a date, always always say yes! It's a bit like auditions, you may not like all the jobs you go up for but you never know when the next one will come along. (Speaking of which, I have a casting for a corporate ad tomorrow, to play "Sophie, a very attractive French business woman. Must speak English with a French accent.")
Example 3: give it your all and more in bed. Afterall, they can't see you in the dark and all they'll remember is what a fantastic time they had. (They always fantasise and imagine you're someone else anyway.)
Example 4: be very very nice. Personality IS everything. Actually, this one applies to ugly and beautiful people. I know a guy who is objectively quite attractive. Problem is, he's self-obsessed and not very generous, and really tight with his money. A bit of a good looking jerk really. Can you imagine what he's like in bed? (especially with the lights off!)
Whether you're plain, it pays to be nice. If you're gorgeous, make sure you keep the lights on...