Use the Smart Phone. I find it disrespectful when someone is constantly using or answering their phone when dining in a group. But when you're on your own, use your phone to your heart's content. It's your entertainment to keep from being bored and takes your attention from the couples or groups dining around you.
Bring a book or newspaper. Like using a smart phone to entertain yourself, reading an actual paper or book should help to pass the time. If it's a buffet the paper or book left on the table lets the staff know you're still eating when returning to the buffet line. But think about keeping up with the time and pick up a Kindle or iPad to always have electronic reading material always ready.
Pony up to the bar. Many patrons who are on the go dine at the bar. Ask for a menu to let the bartender know you plan to eat and drink at the bar. Use this as a last resort or if the restaurant is crowded. Otherwise everyone deserves a table and chair.
Be a food critic. Because I usually take notes while traveling so I remember the "where, when and what" to document my travels, I just make believe I'm a critic reviewing the restaurant. It makes me feel less awkward once I start photographing the food.
See solo diners abound. If you see the restaurant is catering to solo diners, then by all means you know you know you won't be the only one.
Keep it pleasurable. A solo diner's business is as good as any other patron's so relax and enjoy the experience. Choose a restaurant because you want to dine there. The service should not waver just because you're a party of one. Remember the restaurant wants to earn your business and the servers a tip.