1. Dress to impress.
I'm partial to white, but any cute feminine dress will do. Colors are good too, but avoid loud patterns.
2. Wear makeup.
Get your hair done, put on some make-up, whatever you need to do to look like you but... better! When I was pregnant, and doing a self-portrait every two weeks, I realized after the first one or two that I needed to put make-up on. I will never make that mistake again.
3. Wait for good weather.
If possible, when scheduling an engagement portrait, I schedule a shoot date and a rain date. It's not your wedding day, so it doesn't really matter what day you shoot it. I can always make rain work, but if you can, it's worth waiting for a pretty day.
4. Shoot at sunset.
Go for the gold. Wait for the best light of the day.