In general, my advice would be "Don't have regrets." For the most part, there is very little I regret as long as I learned something from it.
We all have skeletons though that we'd rather not dig up ourselves, let alone admit to others. My advice for those is that you can regret them, but forgive yourself for them. I once knew this girl. We shared a bond (no, really), an experience etched in time spanning approximately 48 hours. It wasn't anything sordid; I was 14 at the time. You get to know someone pretty well when you're sitting next to them on a bus for 20 hours having both realized you helped save her life. When you're both 14 there really isn't that much to know.
She'd just gone through a traumatic experience. She had nearly died from dehydration and heat stroke. As I recall, she had lost her stuff when they took her to the hospital. My chaperone had grabbed a sleeping bag at random from under the bus and it happened to be mine. I don't remember clearly why I went to sit next to her (though my sleeping bag was my justification). It may have been my chaperone's suggestion, but I remember she was very pretty. We talked for a long time. Our ages were within a month of each other. We went to different schools about 50 miles apart... but I was moving 500 miles in a matter of weeks.
Until that point in time I was excited about moving, but by the end of the bus ride I didn't want to move anymore. I was infatuated, and she had given me her phone number and address (I think that's the only time that's happened in my life without my own asking). I regret that I was so flustered that when I got to my destination I kind of lost track of her and missed one more thing she later insinuated she wanted to give me; my first kiss.
I've forgiven myself for that last bit, but over the next 5 years we wrote letters back and forth, called each other on occasion, and after I had my license I visited her once or twice and she came to my graduation. I sincerely regret that even as a teenager, I never had the courage to come out and say how I really felt about her; that after she graduated I wanted to see if something more could come of our relationship. I made assumptions and didn't share them. I believed my love was genuine enough. I thought her a wonderful person I enjoyed spending time with. I thought that the things she was involved with were interesting and fun. I loved her as much as any teenager with little life experience can love a girl.
This is where my idea of "aberrant lucidity" comes into play. While I can wish things happened differently, the subsequent heartbreak fostered a more crystallized perspective of reality and kicked me down the path that brought me to where I am today. And while I may not be rich, and I'm just starting out in my career in my early 30s, I like who I am and find value in the things I've learned.
My only regrets are from repeating mistakes from which I recognized the lesson the first time but still duplicated the experiment while expecting different results. Einstein defined that as insanity and I agree. So when you have an aberrant moment of lucidity, embrace it.