For my purposes, weeds fall into one of two categories. Either they are broadleaf weeds, or they are grass type weeds. For both types, they can either be dealt with pre-emergent or post-emergent. Pre-emergent is generally the easiest way to deal with weeds, especially if they appear at certain types of the year. You just put the pre-emergent out at the right time, and it should either wipe them out entirely or at least make them much easier to deal with post-emergent.
If you're dealing with weeds post-emergent, if they fall into the category of broad leaf weeds, they can be dealt with by using Weed-b-Gone, or a similar product. It's good to know the name of the weed so you can check the list of covered weeds. Following the manufacturers recommendations usually produces good results and the product is not that expensive.
For post-emergence of grass type weeds (crabgrass, dallisgrass, paspalum, etc.), the options are a bit trickier. My understanding is that arsenic-based herbicides have been banned by the EPA. I can no longer find MSMA or CSMA anymore so I assume this is true. These were great herbicides because they were cheap and extremely effective post emergent treatment for grassy weeds, especially on southern lawns. Unfortunately golf courses used the stuff by the 55 gallon barrel and it started affecting the water supply in Florida, which is why the EPA banned it. I believe you can still use Quinclorac, but it's not as effective and it's a lot more expensive.
You can always use glyphosate (Roundup) for post-emergence, but obviously this kills both the weed and the grass if both are green. I use glyphosate in the winter to spot treat weeds that are active when my grass is dormant. Glyphosate won't kill dormant grass. When your grass is not dormant, one other strategy is to soak a cloth with glyphosate and wipe it on the leaves of the weed to make the treatment more selective.
My strategy is pretty simple. I use a pre-emergent herbicide in the spring and the fall. I fertilize moderately during the growing season, and I keep my lawn mowed at the right level for the type of grass I have. This keeps most of the weeds at bay and I don't have that much need for post-emergent treatments.
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