I started learning French when I was 29 years old. In my day, foreign language education began in high school and we only had to take two years. In college, my major (microbiology) had no foreign language requirement.
Funny story: I really wanted to take French in high school, but my mom said to me: “You’ll never have any occasion in your life to use French. We live in Arizona. You need to take Spanish.” So I did. Then I married a Frenchman. I love to remind her of this whole event sequence.
After finishing grad school, I met a few French people, fell in love with the language, and decided to take night classes at a small conversation-oriented school to learn how to say “Bonjour” and “Paris” so it would rhyme with “Whee!” A couple immersion programs in France, some private lessons, and a French husband later, and my French is, overall, decent.
Because my “formal” education in French has been minimal, I have holes in my knowledge. Sometimes they are big and gaping, like the issues I have with masculine vs. feminine. Already a difficult concept for the Anglophone, I spent a lot of years whatever-ing the whole thing until I had to admit it actually does matter.
I can’t be the only one who struggles with this in French. Why oh why can’t it be easy, obvious, like it is in Spanish?
Here are a few references I’ve found helpful:
http://french.about.com/od/grammar/a/genderpatterns.htm
http://www.pimsleurapproach.com/resources/french/grammar-guides/masculine-feminine/
http://www.french-linguistics.co.uk/grammar/le_or_la_in_french.shtml
The best/worst advice I’ve (often) received: You just have to learn as you go.
Because there are so many rules and exceptions to rules, learning as you go, from the beginning, is the best route.
I tried to do note cards the other day, based on some of the patterns listed in the websites above. That lasted about 5 minutes before I threw all the note cards I’d worked so hard to create into the recycle bin. Does anyone ever really learn that way? For something other than a cram session for an exam where all the info will be forgotten in a week or two? I’m a complete grammar nerd, but I still recognize a futile attempt when I see it.
Part of my research into bilingualism and teaching languages has shown that our understanding of how best to acquire new languages has evolved greatly, and continues to evolve. It’s much more of an organic process than our typical teaching methods (i.e. rote memorization and recitation) allow for.

I’d rather have my brother stick his foot in my face while I’m posing for a sweet picture with my dad in France than study from notecards.
I need to build associations and habits, not stare at cards. I need to think and speak in French when I go to the grocery store so that I know it’s “la” banane and “le” brocoli. I need to look around my house, think about, talk about the objects I use every day. Le livre. La cuillère. Le four. La table. And so on.
I just want a simple rule or two to follow, but that’s not how French works. It’s going to be a long process before this whole gender thing comes naturally, and accurately, for me.
Yet here are couple that have stuck in my brain for years: problème is masculine and solution is feminine. Hmmm.