“Self, play the mega millions on 11/11/05 when it’s 262 million, here are the numbers. Someone will win it the next draw so play then.”
Bah, I’m not over 45 yet, but if my 45 year old self could come back to now, (well, 6 months ago) I’d tell myself that. 
Your husband is wrong for you. Divorce him while you still have a strong sense of self.
Have as much good sex as you can while the young men still find you attractive.
There’s no getting around the fact that you will have to exercise for the rest of your life.
Don’t be afraid to take risks.
Quit smoking cigarettes.
When you get in a relationship take the time to self-reflect on your behavior.
Make better choices about who you choose for love relationships.
Treat your dog better. I had a butthead of a malamute who was also a great dog. I regret that I didn’t train him better.
Be less worried about what people think of you and try to do more stuff.
Find something better to do with your time than hang out in bars and only spending time with people whose main activity is drinking.
That time when your ex-friend’s ex-wife came to visit you for a weekend? You should have bought a bottle of tequila, got drunk and had great sex. Because that’s what SHE wanted to do. You were too damned polite.
On a less recriminatory note:
The health club membership and losing 30 pounds will pay off. It’s a great feeling when a woman sees you naked for the first time and says, “You have a wonderful body.”
The work you did to maintain friendships with old friends when they became busy with wives and kids or moved out of state paid off. There’s a lot of pleasure in driving down a dirt road, drinking beer and fishing and camping with guys who have known you all your life.
The decision you made to live in a lousy house and be a landlord for seven years will pay off when you decide to buy a new house and can put down a 50% down payment.
It was a good decision to choose celibacy over slutting around. Because when you finally met a good gal you were ready for a real relationship and could say, “Condom, we don’t need no stinking condom!”
You will never regret the vasectomy.
whistlepig
Hey, idiot: stop driving convertibles! They only make you lose your hair faster!
Buy another 944 while they still make them. Porsche will never make another affordable car.
Immediately dump that pretty blond boy that you’ll stupidly be with for the next decade, regardless of how incredibly hot in bed he is! He’ll betray you horribly and you’ll be damned lucky to escape with your health intact.
Most of all: re-learn to play and play well a musical instrument! Your college friend Tim Story taught himself as an adult to play gorgeously from nothing! You know it’s the only thing you’ve ever wanted to do and be.
Don’t break that big toe or shatter that wrist. They are both gonna bother you more and more every year.
Take golf lessons right now instead of waiting 5 years. You’ll enjoy playing better right away.
Spend more time with your mom, dad, and best buddy - they’re gonna be dead soon.