If your diet is crappy you can start with that. Decrease total fat, especially saturated fat, and cholesterol (although dietary cholesterol actually has a pretty small effect). If you're really serious about it, a vegetarian diet is pretty effective. Aerobic exercise is also helpful.
With these changes, most people can reduce LDL-cholesterol by 5-15%. Any more than that, you're probably gonna need drugs.
I keep a strict diet. No cheese, cream of any kind, only skim or non-fat milk on the rare occasions that I have it. Lots of fruit and veg and fish and, as ripsnort says, I go off the wagon about one meal a week--I'm working on decreasing this, though too.
I read the label of every food item I put into my mouth, and make sure to keep saturated fat to about 6-8 grams a day, or about 1/3 that of a normal person. Same with cholesterol. I generally avoid any food where more than 1/3 of the calories are from fat.
I take fish oil tablets, metamucil, and niaspan. Still can't get it below 200. My ratios are good, though. HDL is 46. Still, given my relative youth, and my family history, my doctor isn't happy and I don't blame him.