How large is your lawn?
I would suggest round up every thing, but it is getting late in zone 5 for seeding unless your lawn is small enough to sod!
May need to wait another year!

That's right, it may be too late again this season to help you.

Where do you live? Zone 5 is a big place and has a lot of different soil conditions and climates.
What kind of grass do you want?
How often do you water?
How high do you mow?
How often do you fertilize?

rent a lawn aerator and overseed like crazy and water, water, water! I too used a weed company with little or NO results and then i started following the plan with Scotts and i get almost instant results.

ok this is what i would do. Cut everything as low as u can, throw seed down now and water water water. Dont worry about the weeds. In spring get some weed b gon and spot spray them when they come up - also try thr weed hound its great at getting rid of danelions without chemicals. Then next fall, repeat the overseeding. i did this and now i have 0 weeds!! it works. (if you have time this fall you could also buy/rent a cheap aerator and punch some holes in the ground as well.) good Luck.

Thanks for all this good advice!
We did top dress the lawn during the 1st spring - sod was laid in Fall 2008. We have had the lawn aerated twice since - once last fall and once this spring (2010). We also overseeded in the spring and early summer but maybe didn't water enough...This did help thicken the grass.
I have used the Scott products in another home and found them very good so I think I will try that again.
1. I also usually do not cut the grass very low
2. I water 2-3 times/week
3. We live in Prince Edward County very near Lake Ontario and the top soil is almost non-existent, it is very sandy under the top soil
Thank you!

I would not suggest cutting the grass short and throwing seed around.
Waste of time and money.
In fact, I think you are too late to do much this year!
Probably think about during some work next May.

You are probably mowing just fine.

You are watering far too frequently. Ideally you will let the surface of the soil dry out completely between watering. This usually means you water more deeply when you do water. During the hottest part of the San Antonio summer I water once per week but for 3-5 hours each zone. With my sprinkler and water pressure this puts 3/8 to 5/8 inch of water per week.

Sand is not a problem. I prefer it and bring in sand whenever I need to fix a grading issue.

Top dress? With compost and not with soil/sand I hope.

Seeding should be done in the fall. In your case that means mid August. That gives the seed time to germinate and establish a root system before the heat of summer. I realize your heat is not quite the same as my heat, but it is still heat in the "eyes" of the root system. What ever you do between now and next August, plan to seed again in August.

Another option on seeding is to do a dormant seeding. That involves waiting until it's too cold for the seed to germinate and spreading it. I like to do it just before the first good snow storm. The snow and freeze/thaw cycles bring the seed in contact with the soil and it germinates with the natural spring precipitation as soon as the soil is warm enough to support germination. This gives several weeks head start compared with a "normal" spring seeding.