How to increase HDL Cholesterol

Just got a blood test back that said that I am very low on HDL cholesterol (fortunately, my total is still below 150 but only barely). I'd like to increase my HDL - what are the methods of doing this?

Any particular types of foods (meat, fish, veggies?)

Any particular types of supplements (niacin, vit C, etc)

Just curious what levers I can push on to increase my HDL over time.

Any exercise. When I first went to work on my cholesterol,
I did exercise more but it was mostly diet and both HDL and
LDL went down. When I doubled my workout the LDL kept going down and the HDL went back up although I did nothing to my eating habits.

I've found that it had nearly zero impact. Last year I had blood work done that showed I had vanishingly low HDL, low over-all cholesterol and varies indications of Syndrome X. I decided to change my life and avoid diabetes and heart disease, etc. I got regular with what had been sporadic exercise, added protein to my nearly-vegan diet, started taking
Omega-3 supplements and adding flaxseed to my daily menu and I lost 50 pounds. A year later I had more blood work done. I was astonished at how poorly I had increased my HDL.

BEFORE AFTER
Total Chol: 173 184
HDL: 31 34
RATIO: 5.6 5.4
TRIGL: 121 124

I'm not sure I know why the low HDL is bad. My total cholesterol is supposed to be fine (I'm nearly 39). (I don't know reference ranges for the triglycerides, either.) All I know is that I'm supposed to raise my
HDL. I can try drinking more (it's always been a goal of mine) and I can try the time-release Niacin, but I'd sure like to know why all my efforts in the past year yielded such poor results.

Maybe less effective, but it is damn great for you anyway, so yes please walk instead of the bus.

That exercise impacts HDL sounds counterintuitive to me. How does it work?