Totally agree with veryembarrassed, I'm facing my 40th in a few weeks, and I'm damned if that's going to make me feel old.
Eat well, get as much sleep as possible, find a form of exercise that you enjoy, all of these will make you fell better about yourself.
Get a good haircut, that will take years off you. Likewise get a makeover in one of the big stores to give you ideas, and never leave the house without lipgloss, at the very least.
It's easy to slip into wearing 'middle aged' clothes, read the glossies to keep up with trends.
Wear only heels or converse, NOT trainers or, God forbid crocs.
You are in the prime of your life, don't waste it