Don't skimp on self-development. If you have mediocre skills, that’s strike two. Add a mediocre personality, and it’s usually strike three and you’re out. So self-development, while important for everyone, is crucial for you. Instead of watching TV on the treadmill, read a professional magazine. Instead of loafing around on Saturday morning, go to that seminar on executive deportment. And ask for 360-degree feedback from bosses, co-workers, supervisees and customers.
Make an effort to look sharp. There are some physical attributes we can control and some we can’t. If you ignore the features that you could enhance, it sends the message that you don’t care—a message that may even give the wrong impression of your work. Dress well—at your boss's level of dressiness. Wear a moderate amount of subtle makeup (women only!). Not sure whether yours brings out the best in you? Major department stores have an army of cosmeticians willing to make you over for free.
Don't smoke. If avoiding cancer isn't a good-enough reason, smoking also makes you uglier: It yellows your teeth, wrinkles your skin and makes your breath stink.
Manage your weight as well as you can. Stand up straight. Helen Gurley Brown said, only half joking, “After 40, it all comes down to posture.”
I'd like to add my two cents to these suggestions/this issue:
1. It's a fact of life that attractive people generally get ahead, get the breaks, etc. Don't yell at me -- I didn't set the rules, I'm just recognizing them.
2. That said, it's a shame that it's this way.
3. No matter who you are -- attractive or non-attractive -- you MUST deliver results and have a good attitude. If you don't have these, you're doomed (sure, a good-looking person without these will do better than an unattractive person without them, but he'll still do much worse than most people.)
4. Do whatever you can to make yourself better in all categories -- performance, attitude, AND looks. You need to do everything you can anyway to manage and grow your biggest financial asset (your career).
5. Most people are neither stunning nor ugly, they're in the middle of the pack. As such, most people can simply focus on the basics of looking better (noted above) and this will be good enough.
6. Personally, I cover the basics that make me look at good as possible (proper hygiene, dress appropriately, etc.), but spend the most time on delivering results for my employer as well as being a "yes, I can do that" sort of employee. So far, it's working. :-)