Take apic of the weeds in question. If they are a grassy type weed then it will be harder to get rid of them. Broadleaf weeds are easier to get rid of with herbicides. What type of grass is in your yard? Mike(Texreefer) gave good examples of what you should be doing - good news is it is never to late to start.
I personally do not like to use man made products because of the possibility of cross contamination. Therefore I use mostly natural products. If you keep up with regular mowing, watering, and care to keep the grass healthier then that is the best defense against weeds.
Anyway, if you can get some pics and I will try to help as much as possible.
John
"Critics are men who watch a battle from a high place and then come down and shoot the survivors." Ernest Hemingway