Since a lot of lawn care questions are popping up, here is my attempt to condense all of the basic lawn care advice I have come across. (I'm trying to update this as I think of things or learn more.) This is a general guideline only.

Weeds can grow almost any type of soil, but grass is much more picky. Start by making your soil favorable for grass. Do this stuff at the start of the season (early Spring), or at the end of the season (Fall):

Dethatch when necessary. You might be able to use a dethatching rake, but for a decent-sized lawn you will probably need to rent a dethatching machine, or hire a lawn guy to do it. Get rid of the thatch you pull up (don't just leave it in the yard). Aerate your soil if it is compacted. If water won't sink into your soil, or you can't easily push a spade into the ground, it is probably too compacted. There are multiple types of aerators, but the best one is usually to use a powered core/plug aerator (rent one or hire someone). Aerating will also help to break up your thatch layer, resolving minor thatch problems. Test your soil's pH and adjust it to the proper range. Be sure to test multiple spots in your yard (pH can be high in one spot, but low in another spot). You can buy soil test kits for under $20 at the home improvement stores. You can also send your soil to a lab (check your local university), which can do more extensive testing. Add topsoil or compost to make sure you have lots of healthy microbes. You may need to get a truckload or two of topsoil delivered and use a rake/hoe to spread it around the yard.

The items listed above (usually) don't need to be done every single year. Some lawns can go several years without being dethatched, and still be quite healthy. You may want to rotate through this list each year (dethatch one year, aerate the next, etc). Depends on your specific lawn though.

Keep the grass healthy so weeds don't have an opportunity to take over:

Overseed your lawn to fill in the thin/bare spots. Do this in the late spring and/or early fall (mid-summer is too hot for seedlings). Use the right kind of grass for your climate. If there is no rain in the weather forecast, you may have to water regularly so the seedlings do not dry out. Fertilize the lawn 1-3 times during the season, and don't overdo it (read the instructions on the bag). You may want to consider a "Weed and Feed" product, which is fertilizer mixed with weed treatment (see below). Mow your lawn often enough that you only need to remove 1/3 of the grass height with each mowing. Chopping off too much of the grass at once will stress it. If your lawn is really tall, you may need to mow multiple times, spaced a few days apart, removing 1/3 of the height each time, until you get it down to the desired level. Keep your mower blade sharp. You want to cut the grass cleanly instead of shredding it. You should sharpen the blade at least once a season, more frequently if you have lots of rocks. Keep the mower deck low when the lawn is growing quickly (spring and fall). However, you generally don't want to cut the lawn shorter than 3in. If you mow too short, patches of clover will start to take over and crowd out the grass. Just make sure that 3in is within the rules of your HOA and/or local city/township. Keep the mower deck high when the lawn is growing slowly (summer). In the summer heat, your lawn is under a lot of stress, and mowing too low will make things worse. Just make sure your lawn does not exceed your HOA/city/township rules. Use a mulching mower to return the nutrients back to the soil. Mulch made from your own (healthy) grass is pretty much the perfect fertilizer. So, if you use a mulching mower, you will need to fertilize less often. However, if your lawn is really tall/thick, your mower may struggle to mulch. You shouldn't have clumps of clippings left over. If so, your lawn is too tall (raise the deck, or mow more often) or your mower blade is dull (sharpen it).

Weed Control:

Apply a Weed Preventer (Weed and Feed, Preen, etc) to stop new weeds from appearing, but only if you haven't seeded recently. Weed Preventer also stops new grass from growing. The instructions usually have information about when the product can be applied to recently-seeded lawns. You may need to apply 2-3 times during the season. Use a Weed Spray a few times a year to kill the existing weeds. Be careful about spraying if the lawn was seeded recently (read the label). Also make sure you use a spray that doesn't kill the grass too (don't use Roundup or GroundClear). (See my other answer over here for some tips.) Weed Sprays are usually a mix of a few different herbicides. The mix of herbicide varies from brand to brand (Ortho vs Spectracide) and even among the packaging (hose attachment may have a different formulation than the spray bottle). Some types of weeds are particularly vulnerable to one herbicide, but not others. It helps to identify the offending weed, what kills it, and compare to the list of active ingredients on the weed sprays. Pick the one that will work best for your weeds. Try to stop weeds when they are small. Bigger weeds are more resistant to sprays, so you may have to treat them multiple times, or even have to remove them manually. Killing/removing big weeds will leave holes in the yard that have to be seeded with grass.

Final Tips: